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Chapter 16: “Pre-, syn-, and postcollisional stratigraphic framework and provenance of Upper Triassic–Upper Cretaceous strata in the northwestern Talkeetna Mountains, Alaska” (Hampton et al.), in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O’Neill, J.M., eds., Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska: Geological Society of America Special Paper 431. This PDF file is subject to the following conditions and restrictions: Copyright © 2007, The Geological Society of America, Inc. (GSA). All rights reserved. Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in other subsequent works and to make unlimited copies for noncommercial use in classrooms to further education and science. For any other use, contact Copyright Permissions, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, fax 303-357-1073, editing@geosociety.org. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society.

401

The Geological Society of AmericaSpecial Paper 431

2007

Pre-, syn-, and postcollisional stratigraphic framework and provenance of Upper Triassic–Upper Cretaceous strata

in the northwestern Talkeetna Mountains, Alaska

Brian A. Hampton*Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824-1115, USA

Kenneth D. RidgwayDepartment of Earth & Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-2051, USA

J. Michael O’NeillU.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225, USA

George E. GehrelsDepartment of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

Jeanine SchmidtRobert B. Blodgett

U.S. Geological Survey, 4200 University Dr., Anchorage, Alaska 99508, USA

ABSTRACT

Mesozoic strata of the northwestern Talkeetna Mountains are located in a regionalsuture zone between the allochthonous Wrangellia composite terrane and the formerMesozoic continental margin of North America (i.e., the Yukon-Tanana terrane). New geo-logic mapping, measured stratigraphic sections, and provenance data define a distinctthree-part stratigraphy for these strata. The lowermost unit is greater than 290 m thickand consists of Upper Triassic–Lower Jurassic mafic lavas, fossiliferous limestone, and avolcaniclastic unit that collectively we informally refer to as the Honolulu Pass formation.The uppermost 75 m of the Honolulu Pass formation represent a condensed stratigraphicinterval that records limited sedimentation over a period of up to ca. 25 m.y. during EarlyJurassic time. The contact between the Honolulu Pass formation and the overlying UpperJurassic–Lower Cretaceous clastic marine strata of the Kahiltna assemblage represents aca. 20 m.y. depositional hiatus that spans the Middle Jurassic and part of Late Jurassictime. The Kahiltna assemblage may to be up to 3000 m thick and contains detrital zirconsthat have a robust U-Pb peak probability age of 119.2 Ma (i.e., minimum crystallizationage/maximum depositional age). These data suggest that the upper age of the Kahiltnaassemblage may be a minimum of 10–15 m.y. younger than the previously reported upper

*bhampton@msu.edu

Hampton, B.A., Ridgway, K.D., O’Neill, J.M., Gehrels, G.E., Schmidt, J., and Blodgett, R.B., 2007, Pre-, syn-, and postcollisional stratigraphic framework and prove-nance of Upper Triassic–Upper Cretaceous strata in the northwestern Talkeetna Mountains, Alaska, in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O’Neill, J.M.,eds., Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska: Geological Society of America Special Paper 431, p. 401–438, doi:10.1130/2007.2431(16). For permission to copy, contact editing@geosociety.org. ©2007 The Geological Society of America. All rights reserved.

402 Hampton et al.

INTRODUCTION

Upper Triassic–Cretaceous sedimentary and volcanic strataare exposed between the allochthonous Wrangellia composite ter-rane and the former Mesozoic continental margin of western NorthAmerica in a discontinuous belt that extends from southwesternAlaska to British Columbia (e.g., Berg et al., 1972; Rubin andSaleeby, 1991; McClelland et al., 1992; Cohen et al., 1995; Kappand Gehrels, 1998; Kalbas et al., this volume). Some of the bestexposures of these strata occur in south-central and southwestern

Alaska, where the majority of the exposed Mesozoic stratigraphyconsists of Upper Jurassic–Cretaceous clastic marine strata ofthe Kahiltna assemblage (Fig. 1A). The Kahiltna assemblage andequivalent strata in south-central Alaska occur in an elongate,southwest-trending outcrop belt that is bounded by the Wrangelliacomposite terrane and the Talkeetna fault to the south, and theYukon-Tanana terrane and the Denali and Hines Creek faults to the north (Fig. 1A). This region has been referred to as the AlaskaRange suture zone (Ridgway et al., 2002) or the megasuture zone(Jones et al., 1982). Previous studies have focused on the tectonic

age of Valanginian. Sandstone composition (Q-43% F-30% L-27%—Lv-71% Lm-18%Ls-11%) and U-Pb detrital zircon ages suggest that the Kahiltna assemblage receivedigneous detritus mainly from the active Chisana arc, remnant Chitina and Talkeetna arcs,and Permian–Triassic plutons (Alexander terrane) of the Wrangellia composite terrane.Other sources of detritus for the Kahiltna assemblage were Upper Triassic–Lower Juras-sic plutons of the Taylor Mountains batholith and Devonian–Mississippian plutons; bothof these source areas are part of the Yukon-Tanana terrane. The Kahiltna assemblage isoverlain by previously unrecognized nonmarine strata informally referred to here as theCaribou Pass formation. This unit is at least 250 m thick and has been tentatively assignedan Albian–Cenomanian-to-younger age based on limited palynomorphs and fossil leaves.Sandstone composition (Q-65% F-9% L-26%—Lv-28% Lm-52% Ls-20%) from this unitsuggests a quartz-rich metamorphic source terrane that we interpret as having been theYukon-Tanana terrane. Collectively, provenance data indicate that there was a fundamen-tal shift from mainly arc-related sediment derivation from sources located south of thestudy area during Jurassic–Early Cretaceous (Aptian) time (Kahiltna assemblage) tomainly continental margin-derived sediment from sources located north and east of thestudy area by Albian–Cenomanian time (Caribou Pass formation). We interpret the three-part stratigraphy defined for the northwestern Talkeetna Mountains to represent pre- (theHonolulu Pass formation), syn- (the Kahiltna assemblage), and post- (the Caribou Passformation) collision of the Wrangellia composite terrane with the Mesozoic continentalmargin. A similar Mesozoic stratigraphy appears to exist in other parts of south-centraland southwestern Alaska along the suture zone based on previous regional mapping stud-ies. New geologic mapping utilizing the three-part stratigraphy interprets the northwest-ern Talkeetna Mountains as consisting of two northwest-verging thrust sheets. Ourstructural interpretation is that of more localized thrust-fault imbrication of the three-partstratigraphy in contrast to previous interpretations of nappe emplacement or terranetranslation that require large-scale displacements.

Keywords: Talkeetna, Mesozoic, Stratigraphy, Wrangellia, Kahiltna.

Figure 1. (A) Simplified geologic map of south-central and southwestern Alaska. Note the northeast-southwest trending belt of the Upper Jurassic–Cretaceous Kahiltna assemblage that crops out along the northern margin of the Wrangellia composite terrane and the southern margin of the Yukon-Tanana terrane. In this region of southern Alaska, the Wrangellia composite terrane consists of the Wrangellia and Peninsular terranes. Both of theseoceanic terranes are interpreted to have collided with the Mesozoic continental margin of North America. The Mesozoic continental margin in thestudy area is represented by the Yukon-Tanana terrane. Note that strata in the study area are located in the suture zone between oceanic and continen-tal margin terranes. Map modified from (Nokleberg et al., 1994). Insert in upper left corner shows location of map with respect to Alaska. (B) Moredetailed geologic map showing study area (East Fork Chulitna River area) in the northwestern Talkeetna Mountains. The focus of this study is onUpper Triassic–Lower Jurassic volcanic and sedimentary strata (Tr–J), Upper Jurassic–Lower Cretaceous strata of the Kahiltna assemblage (J–K),and Cretaceous nonmarine strata (K). Data and sample locations on map are from this study. Circled numbers refer to measured stratigraphic sec-tions discussed in the text and shown on Figures 5, 7, and 9.

Cook Inlet

A

Qal

Qal

Qal

TKsa

149º20'W

5

K

v vv

v vv

v vv v

vv v v

STUDY AREA: Northwestern Talkeetna Mtns. - E. Fork Chulitna River area

Qal

K

K

K

J-K

J-K

Pz-Tr

Pz-Tr

Tr-J

Pz

Mz-Cz

Mz-Cz

Tr-J

0 km 100 km

STUDY AREA

N

**

*

*

149º20'W

B R O

A D

P A

S S

63º1

0'N

Qal

East Fork Chulitn

a River

Qal

Mesozoic-Cenozoic (Mz-Cz) - plutonic and volc. rocks (undifferentiated)

U Jurassic-Cretaceous (J-K) - marine sed. rocks – Kahiltna assemblage

Cretaceous (K) - nonmarine and marine sed. rocks – includes nonmarine strata of the Caribou Pass formation in study area and Kuskokwim Group to the southwest

Cret.-Cenozoic (K-Cz and Qal) - sed. and volc. rock (undifferentiated) and alluvium

Triassic- L Jurassic (Tr-J) - volc. and sed. rocks – undifferentiated terranes exposed in the northwestern Talkeetna Mtns. (includes strata of the Honolulu Pass formation in the study area)

Paleozoic (Pz) - sed. and metamorphic rocks – Yukon-Tanana terrane

TTT

Pz-Triassic (Pz-Tr) - sed./metased. and volc. rocks – undifferentiated terranes exposed north of the study area

Triassic-Jurassic (Tr-J) - volc. and sed. rocks – Peninsular terrane

Cretaceous (K) - Albian to younger - Caribou Pass formation – nonmarine sed. rocks containing fossil leaves and Albian-Cenomanian palynomorphs

Penn.-Triassic (Penn-Tr) - volc. and sed. rocks – Wrangellia terrane

Triassic-Jurassic (Tr-J) - sed. and volc. rocks – Chugach subduction complex

This study

DF

DF

BRF

BR

F

HCF

TF

- Hines Creek fault

- Strike-slip fault

- Fault (undifferentiated)

- Border Ranges fault

- Denali fault

HCF

FAULT LEGEND

DF

- Talkeetna fault TF

BRF

LITHOLOGY LEGEND

TF

Penn-Tr

0 km 5 km

4

3

2

1

6

Fossil leaf sample location

Upper Jurassic-Lower Cretaceous (J-K) - Kimmeridgian-Albian - Kahiltna assemblage – marine sed. rocks consisting primarily of turbidite deposits

U-Pb detrital zircon sample location

Megafossil samp. loc. – Buchia fragments

Upper Triassic-Lower Jurassic (Tr-J) - Norian and younger - Honolulu Pass formation – volc. and sed. rocks (pillow basalt, limestone, siltstone, and siliceous mudstone)

Megafossil samp. loc. – bivalve -pectinacean or limoidean?Megafossil samp. loc. – bivalve

-Monotis subcircularisMegafossil samp. loc. – hydrozoan -Heterastridium

J-K

J-J-J-KJ-

J-K

J-KJ-K

J-K

Southernthrust fault

No

rth

ern

thru

st f

ault

Quaternary (Qal) – Alluvium

Cretaceous-Tertiary (K-T) – plut. rocks (granite)

Tertiary (T) – volc. rocks

LITHOLOGY LEGEND

STRUCTURE LEGENDdipping beds

fault -thrust

fault -inferred

fault -undifferentiated

*

B

study

T

Anchorage

Alaska

150º

60º

155º

vertical beds

syncline

TTTTTYYYYTYTTYTTYTTYYYYYTTTTT

N

404 Hampton et al.

setting of the Upper Jurassic–Cretaceous Kahiltna assemblage inthe suture zone (e.g., Pavlis, 1982; Coney and Jones, 1985; Joneset al., 1986; Wallace et al., 1989; Ridgway et al., 2002), however,no previous studies have developed a comprehensive stratigraphicframework for all of the Mesozoic strata in the suture zone.

The purpose of this study is to outline a three-part UpperTriassic–Cretaceous stratigraphy that is observed in the East ForkChulitna River region of the northwestern Talkeetna Mountains(Fig. 1B). We present a newly defined stratigraphy for this regionof the suture zone based on geologic mapping and measuredstratigraphic sections. The study area is one of the few knownlocations in southern Alaska where Upper Triassic–Lower Juras-sic strata are observed in stratigraphic contact with the Kahiltnaassemblage and the first location in south-central Alaska whereCretaceous nonmarine strata have been documented overlyingthe Kahiltna assemblage. We also present new sandstone compo-sitional data and U-Pb detrital zircon ages from the three-partstratigraphy that allow us to identify possible source terranesthat contributed sediment to these units. The detrital zircon agesalso allow us to evaluate the maximum depositional age for theKahiltna assemblage, which prior to this study was based on lim-ited marine macrofossil occurrences. In the final part of the paper,we discuss the regional extent of the new stratigraphic frame-work observed in the northwestern Talkeetna Mountains and dis-cuss how it compares with stratigraphy in other parts of the suturezone throughout south-central and southwestern Alaska.

PREVIOUS WORK

The majority of prior investigations of Mesozoic stratigraphyin the northwestern Talkeetna Mountains have focused mainly onthe significance of these strata within the context of accreted ter-ranes and related overlap assemblages (e.g., Smith, 1927; Joneset al., 1980; Csejtey et al., 1992). Upper Triassic strata in the studyarea have been assigned an age of Norian based on the occurrencesof the bivalve Monotis subcircularis and the hydrozoan Hetera-stridium. Initial documentation of the lithology and age of thesestrata have been summarized in Smith (1927), which presentedlocations and illustrations of Late Triassic fossils in an area locatedsouthwest of our study area. Subsequent studies in the northwest-ern Talkeetna Mountains have resulted in additional fossil localitiesand further lithologic description of Upper Triassic sedimentaryand volcanic strata (fossil localities summarized by Jones et al.,1986, and Csejtey et al., 1992).

The Upper Jurassic–Lower Cretaceous Kahiltna assemblagehas been described within the context of regional geologic map-ping projects throughout south-central and southwestern Alaska(e.g., Csejtey et al., 1978, 1992; Reed and Nelson, 1980; Jones et al., 1982; Smith et al., 1988; Bundtzen et al., 1997), and hasbeen interpreted to represent submarine fan strata that consist ofsandstone, mudstone, and conglomerate with minor interbeddedlimestone (e.g., Wallace et al., 1989; Eastham et al., 2000; Ridg-way et al., 2002). Previous work to determine the age of theKahiltna assemblage throughout the northern part of the Tal-keetna Mountains has been summarized by Csejtey et al. (1992).

A Late Jurassic–Early Cretaceous (Kimmeridgian–Valanginian)age has been assigned to these strata based primarily on the lim-ited occurrence of megafossils that include the bivalves Buchiasublaevis (Jones et al., 1980) and Buchia rugosa (Silberling et al.,1981a, 1981b; Smith et al., 1988). The nearest previously reportedfossil occurrence to the study area is in a Buchia-bearing lime-stone of Valanginian age (e.g., Jones et al., 1980; Csejtey et al.,1992) that is located �35 km to the east. Jones et al. (1980)reported the occurrence of a poorly preserved specimen of theEarly Cretaceous bivalve Inoceramus northwest of the study area.Radiolaria have also been documented in the Kahiltna assem-blage in the northwestern Talkeetna Mountains and suggest aJurassic or Cretaceous age (Jones et al., 1983).

Upper Triassic strata together with the overlying Kahiltnaassemblage have been referred to by previous investigators as theSusitna terrane (Jones et al., 1980, 1981; Silberling et al., 1981a,1981b). The Susitna terrane has been interpreted as an allochtho-nous fault-bounded crustal block that may have undergone signifi-cant northward transport and tectonic juxtaposition to its presentlocation (Jones et al., 1980, 1981; Silberling et al., 1981a, 1981b).Jones et al. (1980) noted the occurrence of Jurassic–Cretaceousstrata both above and below the Upper Triassic strata and inter-preted these relationships to represent either thrust sheets of Tri-assic strata over Jurassic–Cretaceous rocks or an isoclinallyfolded klippe within the Susitna terrane. Csejtey et al. (1992) pro-posed that Upper Triassic–Cretaceous strata are not part of a sep-arate allochthonous terrane but, rather, represent a fault-bounded,overturned klippe containing Upper Triassic and older strata ofWrangellia that have been thrust to the north by a nappe-likestructure that soles southward into the Talkeetna fault (locatedsouth of the study area—see Fig. 1A). Akey component in the lat-ter interpretation is that it requires both large-scale structuralemplacement (>100 km) to account for the present position ofUpper Triassic–Lower Cretaceous strata in this region and that allcontacts between the Triassic strata and the overlying Kahiltnaassemblage be fault contacts.

STRATIGRAPHY

New geologic mapping and detailed measured stratigraphicsections exposed in the East Fork Chulitna River area of thenorthwestern Talkeetna Mountains reveal a distinct three-partstratigraphic framework for Mesozoic strata (Figs. 1, 2). Thisframework consists of a lower unit of Upper Triassic (Norian)–Lower Jurassic marine sedimentary and volcanic strata that werefer to here as the Honolulu Pass formation, a middle unit ofUpper Jurassic–Lower Cretaceous (Kimmeridgian–Aptian) clas-tic marine sedimentary strata known as the Kahiltna assemblage,and an upper unit of Cretaceous (Albian/Cenomanian) to youngernonmarine strata rich in fossil leaves. This upper unit has notbeen described in previous mapping studies of the northwesternTalkeetna Mountains; we informally refer to this unit as the Cari-bou Pass formation. Figure 2 gives an overview of the three-partstratigraphy for this region and summarizes the lithologies and agecontrol (including new data from this study and previous studies).

Figure 2. Chronostratigraphic summary for Upper Triassic–Cretaceous strata of the northwestern Talkeetna Mountains showing new and previouslyreported age control (gray boxes in Age Data column denote extent of macrofossil age ranges, palynomorph ages, and U-Pb detrital zircon ages).Upper Triassic–Lower Cretaceous strata have been assigned ages based on occurrences of marine macrofossils (Smith, 1927; Jones et al., 1986; Cse-jtey et al., 1992), palynomorphs (this study), fossil leaves (this study), and maximum deposition ages determined from U-Pb detrital zircon ages (thisstudy). Note that the uppermost 75 m of the Upper Triassic–Lower Jurassic Honolulu Pass formation is interpreted to consist of a condensed strati-graphic interval that represents limited sedimentation over a period of up to ca. 25 m.y. during Early Jurassic time. Also note the stratigraphic con-tact that represents a ca. 20 m.y. depositional hiatus between conformable strata of the Honolulu Pass formation and overlying Upper Jurassic–LowerCretaceous Kahiltna assemblage. Prior to this study, the youngest age reported for the Kahiltna assemblage was Early Cretaceous (Valanginian); thenew detrital zircon data indicate that the upper age extends at least to Aptian, suggesting that the Kahiltna assemblage is a minimum of 10–15 m.y.younger than previously reported. The Cretaceous to younger Caribou Pass formation has not been reported in previous studies. It is clear that thesestrata overlie the Kahiltna assemblage; however, the nature of this contact has yet to be determined (see text for additional discussion). Note thick-ness of stratigraphic units is not to scale. Time scale based on Gradstein et al. (2004).

C r

e t

a c

e o

u s

J u

r a

s s

i cT

r i

a s

s i c

Late

Ear

lyLa

te

Early

Mid

dle

Late

Ear

lyM

iddl

e65.5

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

251.0

220

240

230

AGE (Ma) AGE DATASTRATIGRAPHY

m s g

Fossil leaves

Palynomorphs

Cenomanian

Norian

BathonianBajocianAalenian

Toarcian

Pliensbachian

Sinemurian

HettangianRhaetian

Callovian

Oxfordian

Valanginian

Kimmeridgian

Albian

Aptian

Barremian

Hauterivian

Berriasian

Tithonian

Maastrichtian

Campanian

SantonianConiacianTuronian

Carnian

Ladinian

Anisian

Olenekian

-gravel (g)-sandstone (s)-mudstone (m)Grain size

m s g

Kahiltna assemblage (marine sed. rocks)

Caribou Pass formation (nonmarine sed. rocks)

?

Upper contact unknown Upper age unknown

Lower contact unknown

Honolulu Pass formation(marine sed. rocks, volcanic -volcaniclastic rocks, and chert sandstone/cong.)

Bivalve - Monotis (Pacimonotis) subcircularis

Hydrozoan - Heterastridium

Bivalve - Buchia sublaevis

* 126.7 Ma

* 124.3 Ma* 117.6 Ma

* 124.5 Ma

FIGURE KEY

Sandstone -tabular; bedded to massive

Siltstone

Mudstone

Limestone

Basalt -tabular sheets - Leaves - Cretaceous

(Cenomanian to younger)rr

- Bivalve - Jurassic (Kimmeridgian-Valanginian)

- Bivalve - Early Jurassic

- Bivalve - Triassic (late Norian)

- Hydrozoan - Triassic (late Norian)

Palynomorph range

U-Pb detrital zircon age (maximum depositional age)

Sandstone -lenticular; cross stratified

Sill

Sandstone/conglomerate -chert grains/pebbles

SEDIMENTARY ROCKS VOLCANIC ROCKS MACROFOSSIL OCCURRENCES

Disconformity

Range of age data occurrences

Basalt -pillows

Dikes

*****

* *

aaaa t i g r a p h i c c o nc i ch p a g r i gt i a t c oo nr a g ri ga t ia a p p hh i c c o nn t a c t )t )c t a ct an t t )c ta c t anD i s c o D ii ss cc ooD i ss cc oo n f o r m i t y ( s t r a (y t ym i tm o r f o n f n ( ss t t r ar mo r f on fn m i t yy ( ss tt r r aaiiii t i o n a l h i a t u s a t i a h i hl a ln ao ni oi t u so n i oi t i n aa ll hh ii aa t u s c a cc a cc aa. 2 0 m. y. d e p o s p oe pd e dy. . y.m.0 m2 0a. 2 o ss0 m2 0a. 2 m.. yy. dd ee pp oo ss

Bivalve - Buchia rugosa

Bivalve - pectinacean or limoidean?

C o n d e n s e d s t r a t i g r a p h i c i n t e r v a l ( 7 5 m d e p o s i t e d i n u p t o 2 5 m. y. )

406 Hampton et al.

D

A

C

100 u

B

D

P

P

P

CPX

CPX

100100 um

100100 um

100

100

Glass

Qm

Honolulu Pass Formation: Upper Triassic (Norian)–Lower Jurassic

Distribution and Measured-Section LocationsUpper Triassic (Norian) sedimentary and volcanic strata con-

sist primarily of interbedded mafic lavas, siliceous volcaniclasticmudstone, fossiliferous limestone (Late Triassic fossils), siltstone,and mudstone, which are overlain by interbedded chert-rich sand-stone and chert-pebble conglomerate, fossiliferous limestone (EarlyJurassic fossils), siltstone, and mudstone (Figs. 3, 4, 5). Thesestrata have been documented in four measured stratigraphic sec-tions (Sections 2, 3, 5, and 6 in Fig. 5). Section 5 is the only mea-

sured section that documents the contact between the HonoluluPass formation and the overlying Kahiltna assemblage (Fig. 5).Our most continuous measured section (290-m-thick Section 2 inFig. 5) does not include the top of the Honolulu Pass formation;therefore, a minimum thickness for the Honolulu Pass formationis 290 m.

Lithologic descriptionLavas. Pillow lavas crop out in continuous sections that are

up to 175 m thick throughout the study area (Fig. 3A; Sections 2and 3 in Fig. 5). Individual beds range from 0.3 to 40 m thick withaverage thicknesses between 3 and 10 m (Fig. 5). Pillow structures

Figure 3. (continued on the next page) Photographs and photomicrographs of Upper Triassic to Lower Jurassic strata that make up the Hono-lulu Pass formation in the study area. (A) Pillow structures common in exposures of mafic lavas of this unit. White lines outline form of indi-vidual pillows. Hammer (white arrow) for scale. (B) Photomicrograph of lava that consists of clinopyroxene (outlined in white with largergrains labeled CPX) within a predominantly plagioclase groundmass. Minor isolated plagioclase laths (P) are up to 250 microns in length. Barscale in right corner. (C) Outcrop showing “tiger-stripe” weathering pattern typical of the siliceous volcaniclastic mudstone lithology. Whitearrow points to hammer for scale. (D) Photomicrograph of siliceous volcaniclastic mudstone unit showing fine-grained matrix that consists ofsilt- and clay-size grains and possibly volcanic glass. Larger grains are mainly chert and quartz. Black arrows point to examples of quartz grains(Qm) and glass shards (labeled “glass”). Bar scale in lower right corner. (E) Limestone in this unit consists of interbedded carbonate mudstone,wackestone, packstone, and grainstone. Resistant beds in photo are wackestone and packstone; recessive beds are grainstone. Beds range inthickness from 0.10 to 0.40 m. Hammer (white arrow) in left center for scale. (F) Photomicrograph of fine- to medium-grained chert sandstonelocated just below the upper 50 m of the formation. Note the poorly sorted, subrounded to angular chert grains (C) and calcite grains (echino-derm fragment outlined by black dashed line) in calcite cement. Bar scale in lower right corner. (G) Clast-supported chert-pebble conglomer-ate that is concentrated in the upper �75 m of this unit. Coin (white arrow) for scale. (H) Photomicrograph of medium-grained chert sandstone,collected 20 m above interbedded fossiliferous limestone and chert sandstone. Note the near monolithic, grain-supported framework consist-ing predominantly of subrounded to rounded chert grains (C) with rare occurrence of calcite grains within a chert-lithic matrix.

Pre-, syn-, and postcollisional stratigraphic framework and provenance of strata 407

are well developed, easily distinguishable by their oblate geome-try (up to 1 m in diameter) and smoothed surfaces (Fig. 3A), andoccur interbedded with tabular beds of massive lava and fossilif-erous calcareous mudstone (Section 2 in Fig. 5). Where pillowstructures are absent, lavas occur as thin, tabular, massive sheets.Vesicles are common in both the massive sheets and in pillowlavas. The dominant constituent in lavas is a fine-grained plagio-clase groundmass (predominantly microcrystalline with rare out-sized plagioclase laths and isolated occurrences of clinopyroxene;Fig. 3B). Measured sections and geologic mapping reveal a lateraltransition from lavas in the northeastern part of the study area tomore fine-grained siliceous volcaniclastic strata in the southwest-ern part of the study area (Sections 2 and 3 to Section 6 in Fig. 5).

Siliceous volcaniclastic mudstone. This unit consists of fine-grained, massive and bedded intervals of siliceous volcaniclasticmudstone strata that commonly exhibit a greenish white andorange banded tiger-stripe pattern on weathered surfaces (Fig.3C). On fresh surfaces siliceous mudstone is dark gray to blackand often exhibits a chert-like appearance. Locally, where this unitis more massive, it has the appearance in outcrop of a fine-grainedvolcanic tuff. Massive intervals are extremely indurated and exhibitconchoidal fracture on unweathered surfaces. Petrographic analy-sis reveals that parts of this unit consist of a fine-grained matrixcomposed of silt- and mud-size grains and possibly volcanic glass,

all of which support isolated silt-sized grains of subrounded chertand quartz (Fig. 3D). Where bedding is evident, the unit is lami-nated, slightly less indurated, and has more of a silty appearance.Due to highly weathered exposures and little change in grain size,individual beds are difficult to distinguish but where observedrange between 0.25 and 10 m thick. Mapping and measured sec-tions suggest that the siliceous volcaniclastic unit is confined tothe upper part of the Honolulu Pass formation and grades laterallyalong strike into massive and pillow lavas. These volcaniclasticstrata and equivalent lavas are some of the most resistant litholo-gies in the study area and make up much of the more pronouncedrelief in the northwestern Talkeetna Mountains.

Limestone. Tabular beds of fossiliferous calcareous mud-stone, which contain the bivalve Monotis subcircularis (Fig. 4A),are interbedded with massive lavas in the upper �150 m of Sec-tion 2 (Fig. 5). Stratigraphically higher in the succession, lime-stone in the uppermost 75 m of the Honolulu Pass formation(Section 5 of Fig. 5) contains fossiliferous limestone (with thebivalve pectinacean or limoidean?) consisting primarily of grain-supported skeletal packstone and grainstone, matrix-supportedskeletal wackestone, and carbonate mudstone interbedded withchert-rich sandstone and conglomerate (Figs. 3E, 4B, 5; carbon-ate classification combined from Dunham, 1962, and Folk, 1959).Individual carbonate beds range in thickness from 6 to 20 cm

BCE

DG

CC

C

C

C

Calcite

Calcite

Cement

Cement

C

C

C

C C

CC

H

F

100

100

100100 um

100100 um

Figure 3. (continued)

Late Triassic (late Norian) Monotis (Pacimonotis) subcircularis

Early Jurassic pectinacean or limoidean?B

A Figure 4. Late Triassic–Early Jurassic agemacrofossils documented from this studyin the Honolulu Pass formation in the EastFork of the Chulitna River area. (A) LateTriassic (late Norian) age bivalve Mono-tis (Pacimonotis) subcircularis from thelower �200 m of measured sections fromthe Honolulu Pass formation. Dashed boxindicates location of enlarged photo andsketch. Ruler for scale. These types offossils commonly occur in carbonate mud-stone beds interbedded with lavas. Theoccurrence of Monotis (Pacimonotis) sub-circularis has been documented in thisregion prior to our study (e.g., Smith,1927; and fossil localities summarized byJones et al., 1986, and Csejtey et al., 1992).(B) Early Jurassic age bivalve pectina-cean or limoidean (?) documented fromthe uppermost 75 m of the Honolulu Passformation. Coin for scale. These types offossils occur in carbonate mudstone bedsinterbedded with grain-supported skele-tal packstone and grainstone, and matrix-supported skeletal wackestone. This isthe first documented occurrence of EarlyJurassic bivalve fossils in the northwest-ern Talkeetna Mountains.

Siltstone

Mudstone

Limestone (Ls)

Sandstone/conglomerate (chert grains/pebbles)

Sandstone

Mafic lavas (massive)

Mafic lavas (pillows)

Mudstone (siliceous)

Bivalve – Late Triassic - late Norian

Bivalve – Late Early Jurassic

Hydrozoan – Late Tr - late NorianSill

Fossil fragments

Stratigraphy continues upsection

H O N O L U L U P A S S F O R M A T I O N

200

0 M

100

s gm

Section 3 Whale Back Ridge N63°06.040' W149°12.018'

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Monotis

subcircularis

Heterastridium

Section 2 E. Fork Chulitna River

structural break

0 Ms gm

N63°10.181' W149°13.678'

N63°10.214' W149°13.610'

Fig. 3B

siliceous volcaniclastic mudstone

100

structural break

Section 6 Antimony Mine

0 Ms gm

N63°06.468' W149°23.045'

Fig. 3D

g0 M

0 M

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100

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smmmM0

mM0 M0 M0 M0000 M0 MM

mMM0

m ggsss ggs gs

Chert-pebble conglomerate andchert sandstone confined to the uppermost 75 m of formation

Kahiltna assemblage(see Fig. 7 for details)

limoidean?

pectinacean or limoidean?

Kahiltna assemblage (base)

Honolulu Pass formation (top)

Section 5 Honolulu Pass

N63°06.250' W149°20.367'

N63°06.367' W149°20.550'

N63°06.250' W149°20.383'

Contact represents at least ca. 20 m.y. during Middle and early Late Jurassic

Disconformity50

25

No fossil occurrences in upper 50 m

Underlain by interbedded basalt, limestone (late Norian), and siliceous volcaniclastic strata

Fig. 3F

Fig. 3H

Condensed stratigraphic interval (upper 75 m)

FIGURE KEY

Figure 5. Four measured stratigraphic sections of Upper Triassic–Lower Jurassic volcanic and marine sedimentary strata of the Hono-lulu Pass formation that document the occurrence of interbedded lava and limestone, siliceous volcaniclastic mudstone, and interbed-ded siltstone and mudstone. See Figure 1B for location of measured sections. Note the stratigraphic occurrence of fossils discussed intext and GPS coordinates for each fossil locality. GPS coordinates at the top of each measured section denote geographic location ofbase of sections. Note the occurrence of Late Triassic (late Norian) fossils in the lower �200 m of Section 2. Section 5 documents theuppermost 75 m of the Honolulu Pass formation and the disconformable contact with the overlying Kahiltna assemblage. The upper 75 m of the formation in Section 5 consist of interbedded chert-rich sandstone, chert-pebble conglomerate, fossiliferous limestone (EarlyJurassic fossils), mudstone, siltstone, and igneous sills (a detailed section of the top of the Honolulu Pass formation is shown to the rightof Section 5). Note the location on measured sections of photomicrographs shown in Figures 3B, 3D, 3F, 3H. We interpret the upper-most 75 m of the Honolulu Pass formation as a condensed stratigraphic interval that represents up to ca. 25 m.y. of limited sedimenta-tion (during Early Jurassic time) that was followed by a ca. 20 m.y. depositional hiatus (spanning the Middle and part of the Late Jurassic)prior to deposition of the Kahiltna assemblage.

410 Hampton et al.

(Fig. 3E).Wackestone and packstone typically form more indurated,resistant beds, whereas grainstone beds exhibit a distinct pock-marked weathering pattern in outcrop (Fig. 3E). Disarticulatedfossil fragments are visible in packstone and grainstone bedsand are typically <2 cm in length. Confident field identificationof fossil fragments is difficult; however, most appear to be bi-valve parts.

Chert-richsandstoneandchert-pebbleconglomerate.Chert-rich sandstone and conglomerate occur in the upper 75 m of theHonolulu Pass formation (Section 5 in Fig. 5), where they areinterbedded with fossiliferous limestone (Early Jurassic fossils),siltstone, mudstone, and igneous sills. Individual sandstone bedsare <2 m thick and consist of medium- to coarse-grained sandstone(Fig. 3F) and are interbedded with beds of conglomerate contain-ing granule- to pebble-size chert clasts (Fig. 3G). Chert-rich sand-stone in the lower part of the 75-m interval (basal �25 m) is fine-to medium-grained and poorly sorted with subrounded to angularchert grains and calcite grains (echinoderm fragments presentlocally) in calcite cement (Fig. 3F). Chert-rich sandstone and con-glomerate in the overlying 50 m, in contrast, are well sorted withwell-rounded grains and clasts. These units are primarily grain-and clast-supported, matrix-free, and absent of sedimentary struc-tures. Petrographic analysis of medium-grained chert-rich sand-stone reveals a near monolithic, grain-supported frameworkconsisting dominantly of subrounded to rounded chert grains withvery rare, isolated occurrences of calcite grains (Fig. 3H).

Age ControlThe oldest strata of the Honolulu Pass formation exposed in

the study area are assigned a Late Triassic (Norian) age based onthe presence of the bivalve Monotis (Pacimonotis) subcircularis(Fig. 4A) and hydrozoan Heterastridium (e.g., Smith, 1927; andfossil localities summarized by Jones et al., 1986; Csejtey et al.,1992). Although occurrences of both Monotis and Heterastridiumhave been documented in previous studies from the northwesternTalkeetna Mountains, we present new fossil localities within thecontext of measured sections. Monotis is described as a thin-shelled, pecten-like bivalve thought to be a pseudoplanktonic andsurface-dwelling organism (Silberling et al., 1997). Occurrences ofMonotis (Pacimonotis) subcircularis in Alaska have been docu-mented from the Wrangellia, Peninsular, Alexander, and NixonFork terranes in southern Alaska, as well as from the North Slope(Silberling et al., 1997). Heterastridium are planktonic and arerecognized by their oblate-spheroid shape and internal radial,cellular structure. Both the bivalve Monotis (Pacimonotis) sub-circularis and hydrozoan Heterastridium correlate with the upperNorian Cordilleranus Zone (Silberling et al., 1997); however,Heterastridium has been reported from older Norian strata fromaround the world.

Fossils that occur in limestone interbedded with chert-pebbleconglomerate in the upper part of the Honolulu Pass formation inSection 5 (Fig. 5) have been tentatively identified as pectinaceanor possibly limoidean (Fig. 4B) and are thought to be Early Juras-sic in age. This is the first reported occurrence of Early Jurassic

pectinacean or limoidean fossils in the northwestern TalkeetnaMountains. In summary, a Late Triassic–Early Jurassic age isinterpreted for the Honolulu Pass formation based on the agerange of all reported fossils.

Interpretation of Depositional EnvironmentsArc-related volcanism. The lavas of the Honolulu Pass for-

mation are interpreted to have formed in a marine setting basedon the presence of pillow structures and interbedded limestonecontaining marine fossils. Volcanic activity was characterized bycoeval subaqueous lava flows and volcaniclastic sediment grav-ity flows. The small lobate geometry of individual pillows and thetabular bed geometries are characteristic of more low-viscositybasaltic or andesitic lava flows rather than high-viscosity rhyoliticlava flows. The occurrence of vesicles throughout the lavas sug-gests the presence of trapped gas and volatiles and implies rapidcooling rather than slower cooling conditions where gas and vola-tiles have a chance to escape the flow. Fossiliferous carbonatemudstone containing Monotis subcircularis and Heterastridiumwere likely deposited during times of subdued volcanic activityor intereruptive periods.

Siliceous volcaniclastic mudstone strata are interpreted asthe products of sediment gravity-flow processes related to pyro-clastic flows (ash-flow tuffs). Deposition was subaqueous; how-ever, pyroclastic flows may have originated from either subaqueouseruptions or subaerial eruptions and subsequent pyroclastic flowinto water. Deposition by pyroclastic flows, when analyzed asdensity-stratified turbidity currents, can result in a wide variety ofsedimentary facies due primarily to variations in flow density(Valentine, 1987) and may account for the variety of sedimentaryand volcanic lithologies observed in this unit. Bedding and crudelaminations are the result of relatively less dense pyroclasticflows, whereas massive, structureless units are the result of high-density turbidity currents. Massive units may also have been theresult of pyroclastic ash fall rather than ash flow, in which casevolcaniclastic detritus was deposited by suspension fallout ratherthan turbidity currents. We tentatively interpret volcanic and vol-caniclastic strata of the Honolulu Pass formation as a product ofarc volcanism; however, detailed geochemical analyses of boththe lavas and volcaniclastic units are needed for a more rigorousinterpretation.

Arc-carbonate platform. Carbonate strata of the HonoluluPass formation are interpreted to have been deposited in low- tohigh-energy environments associated with an immature and poorlydeveloped carbonate platform/ramp. Classification of high-energycarbonate ramp environments includes a shallow inner-ramp en-vironment characterized by constant wave agitation and a mid-ramp environment characterized by frequent storm reworking.Lower-energy ramp environments occur farther offshore andinclude an outer-ramp environment characterized by infrequentsediment reworking and a basinal environment that is character-ized by little to no sediment reworking (e.g., Burchette and Wright,1992). We suggest that limestone beds at the top of the HonoluluPass formation that contain the bivalves pectinacean or limoidean

Pre-, syn-, and postcollisional stratigraphic framework and provenance of strata 411

were likely deposited in high-energy inner- and mid-ramp shore-face environments. The skeletal packstone and grainstone werelikely the result of disarticulation of marine shells during high-energy wave action. Additional support for this interpretationincludes occurrences of bivalves found near the top of the forma-tion that are considered to be shallower-water assemblages com-pared to the bivalve Monotis found in limestone lower in thesection (e.g., Silberling et al., 1997).

The concentration of chert-pebble conglomerate and fossil-iferous chert-rich sandstone in the upper part of the Honolulu Passformation is also likely the result of similar high-energy processesactive within an inner ramp environment. These strata likely rep-resent erosion and reworking of the arc and carbonate platform/ramp. The upsection transition from angular/subrounded grains towell-rounded chert grains in sandstone in the uppermost part ofthe Honolulu Pass formation suggests an environment character-ized by high-energy wave action that resulted in long-termreworking of siliceous volcaniclastic detritus.

The uppermost 75 m of the Honolulu Pass formation areinterpreted as a condensed stratigraphic interval that represents aperiod of limited sedimentation during Early Jurassic time (Fig. 2).We interpret the concentration of well-rounded chert clasts in con-glomerate, chert-rich sandstone, and carbonate grainstone to repre-sent erosion and reworking of arc and carbonate platform strata/sediments in a high-energy inner-ramp environment. The durationof time represented by this condensed interval is unclear, but thedocumentation of Early Jurassic fossils at the base (Section 5 ofFig. 5) suggests that it could potentially record up to ca. 25 m.y.of Early Jurassic time. The top of this condensed interval definesthe depositional contact between the Honolulu Pass formation andthe overlying Kahiltna assemblage (Section 5 of Fig. 5). Theyoungest age-diagnostic fossil in the Honolulu Pass formation isEarly Jurassic (199.6–175.6 Ma), whereas the oldest age diagnos-tic fossil in the Kahiltna assemblage is Late Jurassic (Kimmerid-gian, 155.7–150.8 Ma; discussed in more detail in the followingsection). A disconformity representing a minimum of ca. 20 m.y.between Middle and part of Late Jurassic time, therefore, exists atthe contact between the Honolulu Pass formation and Kahiltnaassemblage. We interpret this ca. 20 m.y. hiatus in deposition as aproduct of subsidence or drowning of the previously active vol-canic arc/carbonate platform that is represented by the HonoluluPass formation. Similar modern examples of subsidence of inac-tive volcanic arc/carbonate platforms have been well documentedfrom the Bismarck arc in the Huon Gulf, Papua New Guinea(Galewsky et al., 1996) and the Hawaiian islands (Moore andFornari, 1984).

Kahiltna Assemblage: Upper Jurassic–Lower Cretaceous(Kimmeridgian–Aptian)

Distribution and Measured-Section LocationsThe Kahiltna assemblage disconformably overlies the Hono-

lulu Pass formation in the study area and is made up primarily oftabular beds of sandstone, siltstone, and mudstone (Figs. 6, 7).

This unit is estimated to be up to 3000 m thick in parts of south-central Alaska, but a single continuous section has not beendocumented in our study area. Our Section 5 covers the lower�575 m of the unit that directly overlie the Honolulu Pass forma-tion (Fig. 7), and we also measured several sections from higherin the Kahiltna assemblage (Sections 2, 4, and 6 of Fig. 7). Ourmeasured sections show that the Kahiltna assemblage coarsensupward from predominantly mudstone near the base to moresandstone-rich upsection (Fig. 7).

Lithologic DescriptionSandstone, siltstone, and mudstone (subordinate conglom-

erate). The Kahiltna assemblage is characterized by dark- to light-gray siltstone, fine- to medium-grained sandstone, and subordinatemudstone (Figs. 6A, 6B, 7). The lower part of the Kahiltna as-semblage is defined predominantly by interbedded laminated tomassive mudstone and siltstone where individual beds are typi-cally <0.2 m thick (Section 5 in Fig. 7). Individual beds fineupward from siltstone to mudstone. The sandstone beds that formthe upper part of the Kahiltna assemblage (Sections 2, 4, and 6 inFig. 7) are typically 0.1–1 m thick (Figs. 6A, 6B). Isolated sand-stone beds up to 3 m thick occur in the upper part of the Kahiltnaassemblage. Sandstone units are tabular with no evidence of basalerosional scour (Figs. 6A, 6B). Individual sandstone units havesharp basal contacts and transitional upper contacts that com-monly grade into siltstone. Sedimentary structures in the sand-stone consist primarily of horizontal stratification and ripplecross-stratification (Fig. 6C). A typical vertical distribution offacies within an individual sandstone unit consists of massivesandstone that grades upward into horizontally stratified sand-stone with ripple cross-stratified sandstone and siltstone at the topof the bed (Fig. 7). Sandstone units are either directly overlain bymudstone or, in some cases, by siltstone that grades upward intomudstone. Climbing-ripple structures (Fig. 6C) and distorted,convolute bedding are common in individual sandstone units.Load structures have been observed at the base of individual beds.Rare tabular beds of conglomerate (up to 1 m thick) are matrix-supported with clast sizes typically <2 cm in diameter. Individualconglomerate clasts are subrounded to well rounded and consistprimarily of gray chert, dark gray siltstone, and quartz.

Age ControlThe Kahiltna assemblage in the northwestern Talkeetna

Mountains has been assigned an age of Late Jurassic (Kimmerid-gian) to Early Cretaceous (Valanginian) based on previous stud-ies that have documented the occurrence of the bivalve Buchiasublaevis and Buchia rugosa (Jones et al., 1980; Silberling et al.,1981a; Smith et al., 1988). This biostratigraphic age assignmentsuggests that the Kahiltna assemblage was deposited between 155and 137 Ma. It is important to note that fossil control is limited forthe Kahiltna assemblage in the northwestern Talkeetna Moun-tains (fewer than five fossil localities). In a later section, we pre-sent new U-Pb detrital zircon ages for the Kahiltna assemblagefrom sandstone samples collected within the context of each of

412 Hampton et al.

our measured sections (Fig. 7). The youngest detrital zircon agesfall between 130 and 115 Ma and indicate that the upper age ofthe Kahiltna assemblage in the East Fork Chulitna River area isat least as young as latest Early Cretaceous (Aptian). Our detritalzircon data imply that the Kahiltna assemblage is, at a minimum,10–15 m.y. younger than the previously reported Valanginian age.We present a detailed summary of the U-Pb detrital zircon age dis-tribution for the Kahiltna assemblage and review possible sourceareas for these grains in a later part of this paper.

Interpretation of Depositional EnvironmentsSubmarine fan system. The Kahiltna assemblage exposed

in the East Fork Chulitna River area is interpreted to represent depo-sition by a combination of low- to high-density flow events associ-ated with sediment gravity flows in submarine fan environments.The occurrence of both laminated mudstone and siltstone and hori-zontally stratified sandstone together with massive sandstone andripple cross-stratified sandstone suggests sediment distribution wasinfluenced by both laminar and turbulent flow. Tabular beds marked

A

C

B

Figure 6. Photographs of the Upper Jurassic–Lower Cretaceous (Kimmeridgian–Aptian) Kahiltna assemblage in the study area. (A) Tabular bedsof sandstone, siltstone, and mudstone that characterize this unit. White arrow points to person for scale. Bedding dips steeply to the right. (B) Thelower part of the Kahiltna assemblage consists mainly of fine-grained sandstone and siltstone in tabular beds that are typically between 0.1 and1 m thick. Hammer (white arrow) for scale. (C) Common sedimentary structures in the Kahiltna assemblage include climbing-ripple stratifica-tion and convolute bedding. Black camera lens cap (lower left) for scale.

Figure 7. Four measured stratigraphic sections of the Kahiltna assemblage. See Figure 1B for location of measured sections. GPS coordinates denotegeographic location of base of measured sections. Detailed stratigraphy from Sections 4 and 5 shows vertical distribution of sandstone facies. Thelower �575 m of the Kahiltna assemblage is documented in Section 5, and individual beds show a common distribution of facies that includes hori-zontally stratified and ripple cross-stratified sandstone along with interspersed convolute beds. Soft-sediment deformation and climbing-ripple strati-fication are more common in the upper part of the Kahiltna assemblage. Section 4 shows how much thicker (up to 3 m thick in places) and morecoarse-grained individual sandstone units become upsection in the Kahiltna assemblage; compare fine-grained strata of Section 5 to Sections 2, 4, 6.Section 4 shows a vertical section consisting predominantly of massive sandstone (Sm), and horizontally stratified and ripple cross-stratified sand-stone interbedded with subordinate siltstone. Note the location of U-Pb detrital zircon samples that are discussed in the text (ages reported representminimum crystallization ages/maximum depositional ages).

0 M

100

200

s gm

0 M

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s gm

200

300

124.5 Ma

Section 4 Three Cirque Basin

Section 6 Antimony Mine

structural break

300

400

* 126.7 Ma

- Bivalves – Early JurassicSiltstone

Mudstone

Limestone

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Sandstone Sill

- Ripple-cross stratification

- Horizontal stratification

Sm = Massive sandstone

(Sr)

(Sh)

- Soft-sediment deformation

K A H I L T N A A S S E M B L A G E

Sm

Sm

Sm

Sm

Sm 15

20

** *

SrSh

Sandstone/conglomerate (chert grains/pebbles)

* Stratigraphic location of U-Pb detrital zircon sample (age denotes maximum depositional age)*

0 M

100

s gm

200

* 117.6 Ma

Section 2 E. Fork Chulitna River

structural break

*

N63°10.181' W149°13.678'

N63°06.468' W149°23.045'

N63°12.274' W149°07.712'

300

400

700

500

600* 124.3 Ma

Section 5 Honolulu Pass

Sm627

628

*

Sr

Sr

Sr

Sh

Sh

Sh

Sh

N63°06.250' W149°20.367'

Kahiltna assemblage (base)

Honolulu Pass formation (top)

Condensed stratigraphic interval (upper 75 m)

Contact represents up to ca. 20 m.y. during Middle and early Late Jurassic

g0 M

100

200

sm

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er 7755 mm))m5 m)5 mc aphic

)ra5rap ic phra hicaph ch chich

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M0 M00 MM00 M

Represents up to ca. 25 m.y during Early Jurassic time

Refer to Fig. 5 for detailed stratigraphy

Underlain by interbedded basalt, limestone (late Norian), and siliceous volcaniclastic strata

Stratigraphy continues upsection

Disconformity

414 Hampton et al.

by sharp nonerosive basal contacts support laminar flow in the lowerportions of sediment gravity flows. The presence of climbing-ripplestructures implies deposition in medium- to high-density flows char-acterized by a high suspended sediment load (Lowe, 1982). Mas-sive mudstone and siltstone were likely associated with the waningstages of individual flow events (e.g., Ghibaudo, 1992). Convolutebedding is the result of postdepositional reorganization throughdewatering and likely occurred in strata associated with high sus-pended sediment load during original deposition.

The upward-coarsening trend from predominantly mudstoneand siltstone near the base (Section 5 in Fig. 7) to increased sand-stone upsection (Sections 2, 4, 6 in Fig. 7) suggests an overallprogradational package where younger and more proximal partsof submarine fan systems successively prograded over older,more distal, finer-grained submarine fan deposits or slope tur-bidites that make up the base of the Kahiltna assemblage. Ourdescriptions of the upper and lower portions of the Kahiltnaassemblage are consistent with previous interpretations of thisunit as representing submarine fan deposition (Csejtey et al., 1992;Eastham and Ridgway, 2002; Ridgway et al., 2002).

Caribou Pass Formation: Cretaceous (Albian/Campanianto Younger)

The uppermost unit of the three-part stratigraphy in the studyarea consists primarily of interbedded fine- to medium-grainedsandstone and fossil leaf-bearing siltstone and black mudstone withlimited occurrences of clast-supported conglomerate (Figs. 8, 9).The minimum thickness for this unit is �250 m, documentedfrom a measured section from the northeastern part of the studyarea (Section 1 in Fig. 9). Overall, the Caribou Pass formation rep-resents the most coarse-grained strata observed in the study area.

Lithologic DescriptionSandstone, siltstone, and mudstone (subordinate conglomer-

ate). Sandstone units of the Caribou Pass formation have lenticu-lar geometries and are characterized by erosional scour contactsand mudstone rip-up clasts near their base. Individual sandstonebodies consist of vertically stacked beds that are typically between0.2 and 1 m thick (Figs. 8A, 9) and are up to 3 m thick in places.Low-angle, sloped surfaces are common at the base of some sand-stone beds and are referred to in our descriptions as lateral accretionsurfaces (Figs. 8A, 8A�, and 9). A common vertical distributionof sedimentary structures in sandstone beds includes basal planarcross-stratification overlain by trough cross-stratification andcapped by horizontal stratification or ripple cross-stratification(Figs. 8B, 9). Distorted, convolute bedding was documented locallythroughout this unit (Fig. 9). Conglomerate beds are typically<0.5 m thick and are well sorted, sandy, and often appear massive(Figs. 8C). Clasts range in size from 0.5 to 1.5 cm and consistmainly of rounded black and gray chert and quartz clasts. Minorsubrounded, elongate clasts of siltstone and mudstone also occur.

Siltstone and mudstone form successions up to 10 m thick inthe Caribou Pass formation (Fig. 9). Individual beds are typically

<1 m thick, tabular, and consist of laminated mudstone and mas-sive to laminated siltstone. The basal part of these finer-grainedbeds is characterized by a sharp contact with underlying sand-stone units. Upper contacts are often scoured by overlying sand-stone units. Fossil leaves ranging from 4 to 10 cm in length occurprimarily within very fine-grained sandstone, massive siltstone,and laminated mudstone (Figs. 8D, 8D�). The best-preserved fos-sil leaves occur in siltstone and represent leaf mats containingnumerous overlapping leaf traces. Individual fossil leaves appearto have a graphite-like carbon film, and their host siltstone ormudstone commonly gives off a phyllitic sheen. Fossil ferns orpossibly conifer foliage also occurs locally on bedding planeswithin finely laminated mudstone. Elongated wood fragments (upto 15 cm long) and individual seeds or pods (<4 cm in diameter)occur sporadically throughout mudstone, siltstone, and sandstonein this unit.

Age ControlA total of ten samples were collected from the Caribou Pass

formation for palynomorph analysis. Recovery from three of thesesamples, representing one sample location, yielded occurrencesof Polycingulatisporites reduncus (gymnosperm pollen) and Cica-tricosisporites cf. venustus (Pteridophytic spores), both of whichindicate that this unit could be as old as Cretaceous (Albian–Cenomanian; Ravn, 1995). Figure 10 provides a list of palyno-morphs found from this unit. The upper age limit of these stratais unknown but is loosely constrained to Cenomanian or youngerbased on the occurrence of leaf fossils. The possible occurrenceof several platanoid leaf fossils may suggest an uppermost EarlyCretaceous to Late Cretaceous age (Albian–Cenomanian), but theleaves could also be as young as Paleocene age (Scott Wing, 2006,personal commun.). We tentatively suggest that the Caribou Passformation is Albian/Cenomanian to younger based on the agerange of palynomorphs and the occurrence of fossil leaves.

Interpretation of Depositional EnvironmentsFluvial system. The Caribou Pass formation is interpreted to

be the result of traction transport in confined fluvial channels andof laminar flow in overbank floodplain regions. The presence oflenticular sandstone units, lateral accretion surfaces, planar cross-stratification, and trough cross-stratification is indicative of sig-nificant sediment migration by lateral and downstream barswithin fluvial channels (Miall, 1978, 1985). Basal erosional scoursurfaces and rip-up clasts suggest that channel flow velocitieswere sufficient to scour and erode underlying units. Convolutebedding may have resulted from dewatering of beds after periodsof deposition in bedload conditions in channels. Deposition of con-glomerate suggests sedimentation during periods of upper flowregime conditions. Tabular, laminated beds of mudstone and silt-stone are interpreted to represent unconfined sedimentation inoverbank floodplain regions. The preservation of fossil leavesin these strata suggest that floodplain regions were stable anddefined by periods of nondeposition at least long enough to pro-mote the development of vegetation.

A

D

A'

ShSh

StSt

SpSp

B

D'

C

GmGm

SmSm

GmGm

Figure 8. Photographs of the Cretaceous (Albian/Cenomanian) to younger nonmarine Caribou Pass formation. (A) Lateral accretion surfacesin sandstone. Hammer (white arrow) for scale. (A�) Identical photo, but with bold white lines illustrating low-angle lateral accretion surfaces(thin white lines represent base and top of low-angle bedsets). Note how bed thicknesses pinch out laterally in these units (from left to rightside of photo). Hammer (white arrow) for scale. (B) Planar cross-stratified sandstone (Sp) overlain by low-angle trough cross-stratified sand-stone (St) and capped by horizontally stratified sandstone (Sh). Black camera lens cap (white arrow) for scale. (C) Rounded- to well-rounded,moderate- to well-sorted, clast-supported conglomerate (Gm) with sandstone interbeds (Sm). Bedding dips gently to the right side of the photo.Ruler is 15 cm long for scale. Individual clasts range from 0.5 to 1.5 cm in a typical conglomerate. (D) Fossil leaf imprints on the top of a silt-stone bed. White arrow shows coin for scale. (D�) Identical photo, but with white lines outlining fossil leaf margins.

416 Hampton et al.

- Ripple cross- stratification (Sr)

- Horizontal stratification (Sh)

- Soft-sediment deformation

- Trough cross- stratification (St)

- Fossil leaves

Conglomerate - Planar cross- stratification (Sp)

LAS = Lateral accretion surfaces Gm = Gravel (massive)

0 M

100

200

300

s gm

Fossil leaves

C A R I B O U P A S S F O R M A T I O N

LAS

LAS

Gm

Sp

Sr

Sr

Sh

Sp

Sh

St

Sh

St

29

30

31

Section 1 Fossil Leaf Ridge

N63°12.274' W149°07.712'

N63°12.274' W149°07.712'

Stratigraphy continues upsectionSiltstone

Mudstone

Sandstone

Figure 9. Measured stratigraphic section of the Cretaceous (Albian/Cenomanian) to younger nonmarine Caribou Pass formation from Section 1.See Figure 1B for location of measured section. Note the stratigraphic position of fossil leaves discussed in text and GPS coordinates of fos-sil locality. GPS coordinates at the top of measured section denote geographic location of base of measured section. A total of 252 m of non-marine strata is represented in this measured section. Individual sandstone units are often lenticular, typically 1–2 m thick, and consist ofcross-stratified, medium-grained sandstone. Lateral accretion surfaces, soft sediment deformation, and basal erosional scour surfaces are com-mon. Fossil leaves occur in interbedded very fine-grained sandstone, siltstone, and mudstone units.

STRATIGRAPHIC AND STRUCTURALRELATIONSHIPS

Our geologic mapping and measured stratigraphic sectionsdocument several geologic relationships that have not been re-ported by previous studies. In this section, we describe two keystratigraphic contacts and present a revised structural interpreta-tion for the northwestern Talkeetna Mountains.

Stratigraphic Configuration

Contact 1: Top of the Honolulu Pass Formation—Base ofthe Kahiltna Assemblage

Our stratigraphic data document a disconformable depositionalcontact between the top of the Honolulu Pass formation and the base

of the Kahiltna assemblage (Section 5 in Figs. 5, 7). New map-ping from our study suggests that both units are part of a continuousstratigraphic package that has been incorporated into two northwest-verging thrust sheets (Figs. 11A, 11B, 12A). At a distance, this rela-tionship is recognized by a change from gray to yellow, massiveresistant units of the Honolulu Pass formation (labeled Tr–J on Fig. 11C) to less-resistant dark black units of the overlying Kahiltnaassemblage (labeled J–K on Fig. 11C). The contact between the twounits is located at the top of an �75-m-thick interval of interbeddedsiltstone, mudstone, chert-rich sandstone, chert-pebble conglomer-ate, fossiliferous limestone (Early Jurassic age fossils), and igneoussills that define the top of the Honolulu Pass formation (Figs. 5,11D). This interval is overlain by mudstone of the Kahiltna assem-blage (J–K; Section 5 in Fig. 5, Figs. 11C, 11D). We found no evi-dence of a structural break across this contact.

Palynomorph type and age range

99.6

Cenomanian

Albian

112

93.5

Del

toid

ospo

ra s

pp.

Laev

igat

ospo

rites

sp.

Aeg

uitr

iradi

tes

spin

ulos

us

Osm

unda

cidi

tes

wel

lman

ii

Dis

taltr

iang

ulis

porit

es p

erpl

exus

Plic

atel

la c

rista

ta

Ret

itrile

tes

sp.

Per

inop

olle

nite

s cf

. ela

toid

es

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uitr

iradi

tes

orna

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iche

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ites

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nicu

s

Odo

ntoc

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a op

ercu

lata

Cic

atric

osis

porit

es v

enus

tus

Pol

ycin

gula

tispo

rites

red

uncu

s

Ma

Ma

Ma

Figure 10. Age range of Albian–Cenomanian palynomorphs of the Caribou Pass formation. A maximum age of Albian is assigned tothis unit based on palynomorph assemblages. Geologic time scale from Albian to Cenomanian (112 Ma to 93.5 Ma) is based on Grad-stein et al. (2004).

AAA

K

K

KTr-JTr-J

J-K

J-K

J-K J-K

J-K

Figure 11. (continued on the next page) Photographs of geologic relationships mapped in the East Fork Chulitna River area of the northwesternTalkeetna Mountains. White dip-direction indicators show orientation of beds in each photo. (A) Regional view from the northeasternmost partof the field area looking southwestward across the East Fork of the Chulitna River (valley in midground) at the deformed three-part stratigraphicsuccession. A southeast-dipping thrust fault (dashed line) carries Upper Triassic–Lower Jurassic Honolulu Pass formation (Tr–J), and UpperJurassic–Lower Cretaceous strata of the Kahiltna assemblage (J–K) in the hanging wall and juxtaposes them against Kahiltna assemblage in thefootwall (northern thrust fault of Fig. 1B). Note that Cretaceous to younger strata of the Caribou Pass formation (K) overlie the Kahiltna assem-blage in the footwall (right part of photo). The Caribou Pass formation (K) has been deformed into a northeast-trending syncline in the right halfof the photo. The broken thin white lines in the center of the photo mark a series of sills that define the boundary between the Kahiltna assem-blage and the Caribou Pass formation. A close-up photo of this relationship is shown in Figure 11E. (B) Two north-verging thrust sheets in thefield area (northern and southern thrust faults of Fig. 1B). View to the south-southwest. Note that the Honolulu Pass formation (Tr–J) has a depo-sition contact with the overlying Kahiltna assemblage (J–K) in both thrust sheets. Dashed white lines show location of the two thrust faults andarrows show direction of hanging wall displacement. Note that tectonic transport is to the northwest. White circle in lower left part of the photooutlines two tents for scale. (C) Depositional contact between the top of the Honolulu Pass formation (Tr–J) and overlying Kahiltna assemblage(J–K) that can be recognized by a break in color between the gray and yellow resistant layers of the Honolulu Pass formation (lower part of photo)and dark gray to black, less resistant strata of the Kahiltna assemblage. Dashed white line indicates approximate location of the contact. Expo-sure is �500 m from base to top of photo. (D) Photo showing the depositional contact between the Honolulu Pass formation (Tr–J) and Kahiltnaassemblage (J–K). Fossiliferous limestone of the Honolulu Pass formation (Tr–J) is exposed in the rightmost portion of the photo, and beddingis dipping to the left. The base of the Kahiltna assemblage (J–K) is exposed in the left part of photo. A 75-m-thick succession of interbeddedchert-pebble conglomerate, fossiliferous limestone (Early Jurassic age fossils), siltstone, mudstone, and igneous sills makes up the top of theHonolulu Pass formation and represents up to ca. 25 m.y. of limited sedimentation during the Early Jurassic. The top of the Honolulu Pass for-mation is defined by a disconformable contact that represents ca. 20 m.y. of nondeposition during Middle and part of Late Jurassic time. Thereis no change in bedding orientation across the contact and no evidence for a structural break. (E) An angular unconformity or fault contactobserved along the southeastern limb of the syncline between the Caribou Pass formation (K; upper part of photo) and the Kahiltna assemblage(J–K; lower part of photo). Note that the contact between these units is marked by a poorly exposed zone >200 m thick that contains a series ofigneous dikes and sills. Dashed white lines mark locations of the more obvious sills. The nature of this contact is poorly understood and willrequire additional mapping. View is to the northeast. See Figure 12A for location of this photo in the context of structural location.

418 Hampton et al.

TrJr-KTr Jr-K

K

B

Tr-J

J-K

J-K

Jr-K

J-K

Tr

CC

Tr-J

J-K

ED

J-K

Tr-J

Tr-J

J-K

K

KK

Tr-J

Figure 12. Comparison of new detailed geologic mapping and the most recent previous interpretation for the study area based on regional mapping.(A) Geologic map and cross section based on new mapping and measured sections in the study area showing a depositional contact between the Hono-lulu Pass formation (Tr–J) and the overlying Kahiltna assemblage (J–K) exposed in the hanging wall of two northwest-verging thrust faults. Notethat in this interpretation, the Caribou Pass formation and Kahiltna assemblage are located in the footwall of the northernmost thrust fault. (B) Pre-vious map and cross section for this area from Csejtey et al. (1992) that interprets the Upper Triassic–Lower Cretaceous strata as having been struc-turally emplaced. This interpretation requires that every contact between the Honolulu Pass formation and the Kahiltna assemblage be a fault contact.In this interpretation, the Honolulu Pass formation represents an allochthonous rootless nappe that has been tectonically transported long distances.The Caribou Pass formation has not been documented in previous studies.

Contact 2: Top of the Kahiltna Assemblage—Base of theCaribou Pass Formation

The contact between the Kahiltna assemblage and the CaribouPass formation is best exposed along the northeast trending syn-cline located north of the East Fork of the Chulitna River (Fig. 12A).Along the southeastern margin of the syncline, the contact isdefined by a diffuse >200-m-thick zone of igneous sills and dikes(Fig. 11E). Directly under this poorly exposed zone, beds of theKahiltna assemblage (J–K on Figs. 11E, 12) are vertical to over-turned, whereas, above the diffuse zone, beds of the Caribou Passformation dip gently to the northwest (K on Figs. 11E, 12A). Thiscontact is exposed �1 km from a fault that is mapped in theKahiltna assemblage (Fig. 12A). It is not clear whether the abruptchange in bedding across the diffuse zone represents an angular

unconformity between the Kahiltna assemblage and the CaribouPass formation or if it is a fault contact. Along the northwesternmargin of the syncline, the contact appears to be depositional withboth the Kahiltna assemblage and the Caribou Pass formation dip-ping to the southeast (Fig. 12A). Studies are ongoing to furtherunderstand the nature of the contact between the Kahiltna assem-blage and the Caribou Pass formation in the northwestern TalkeetnaMountains (Altekruse et al., 2006).

Previous Structural Interpretation

Previous studies have suggested that the Upper Triassic–Cretaceous strata in the northwestern Talkeetna Mountains rep-resent an allochthonous, fault-bounded, crustal block that may

Figure 11. (continued)

QalQal Qal

A'A

3

0

-3

N E W M A P P I N G P R E V I O U S M A P P I N G

OTOT OT

J-K J-KJ-K

J-K

3

0

-3Km

3

0

-3Km

A'A

Fault-bounded stratigraphy

Fault-bounded stratigraphy

Northernthrust fault

A'A

A'

A149º20'W

B R O

A D

P A

S S

K

0 km 5 km

J-K

J-K

J-K

N E W I N T E R P R E T A T I O N P R E V I O U S I N T E R P R E T A T I O N

OT

Cretaceous (K) -Albian to younger -Caribou Pass formation – nonmarine sed. rocks containing fossil leaves and Albian- Cenomanian palynomorphs

Upper Jurassic-Lower Cretaceous (J-K) - Kimmeridgian-Albian - Kahiltna assemblage – marine sed. rocks consisting primarily of turbidite deposits

Upper Triassic-Lower Jurassic (Tr-J) - Norian-Lower Jurassic - Honolulu Pass formation – volc. and sed. rocks, pillow basalt, limestone, siltstone, and siliceous mudstone

QuaternaryQuaternary (Qal) Alluvium(Qal) – Alluvium

Cretaceous-Tertiary. (K-T) – plut. rocks (granite) -Paleocene-Oligocene

Tertiary (T) – volc. rocks (basalt) -Paleocene-Oligocene

Thisstudy

Dipping beds

Thrust fault

Bedding

Fault -thrust

Fault -inferred

Fault -undifferentiated

Thrust fault (overturned/observed)

Anticline (overturned/observed)

Syncline (overturned/inferred)

Bedding(overturned)

B

North South North South

63º1

0'N

Qal

Qal

Qal

Qal

T

Vertical beds Overturnedbeds (inferred)

Southern thrust fault

Fault-boundedrootless nappe

Fault-bounded Fault boundedrootless nappe rootless nappe

Qal

Qal

Qal

v

TKsaK

vv vv vv vv vv vv vv vv vv

v

vv

v

vv

v vv vv

v vv v

Qal

East Fork Chulitn

a River

Qal

3

J-K

J-J-J-KJ-

J-K

J-K

T

Nor

ther

nth

rust

faul

t

B R O

A D

P A

S S

0 km 5 km

A'

A

A

Southern thrust fault

149º20'W

63º1

0'N

FIGURE KEY

420 Hampton et al.

TABLE 1. SUMMARY OF PARAMETERSFOR SANDSTONE POINT COUNTS

—Quartz (Q) � Qm � Qp � chert

—Monocrystalline quartz (Qm)—Polycrystalline quartz (Qp)—Chert (C)

–Feldspar (F) � P � K

—Plagioclase (P)—Potassium feldspar (K)

-Lithic fragments (L) � Ls � Lm � Lv

—Lithic sedimentary (Ls)—Argillite (Lsa)—Mudstone (Lsmd)—Sandstone (Lssn)—Limestone (Lslm)

—Lithic metamorphic (Lm)—Phyllite (Lmph)—Shist (Lmsh)—Gneiss (Lmgn)—Metachert (Lmc)—Metavolcanic (Lmv)

—Lithic volcanic (Lv)

Lt � Lv � Ls � Lm � Qp � chert

Lvm � Lv � metavolcanic

Lsm � Ls � metasedimentary

have undergone significant displacement and deformation duringnorthward transport and juxtaposition along the continental marginof North America (Susitna terrane of Jones et al., 1980, 1981; Sil-berling et al., 1981a, 1981b). Other mapping studies have inter-preted the Honolulu Pass formation and the Kahiltna assemblage as part of a series of overturned, structurally emplaced, rootlessnappes that ultimately restore back to Wrangellia and the Talkeetnafault to the south (Figs. 1A, 12B; e.g., Csejtey et al., 1992). Thisinterpretation requires that part of the stratigraphy in the study areabe overturned and that every contact between the Honolulu Passformation and the Kahiltna assemblage be a fault (Fig. 12B). Morerecent studies of deformation fabrics and facies distribution alongthe Talkeetna fault have suggested that the fault may not representa large-scale structure or terrane boundary (O’Neill et al., 2003).

Revised Structural Interpretation

Figure 12 provides a comparison of previous mapping in thenorthwestern Talkeetna Mountains with our new mapping data.We observe that the majority of the Honolulu Pass formation andKahiltna assemblage consist of upright, tilted strata that dip south-eastward. Typical southeast-dipping beds of the Honolulu Passformation (Tr–J) and Kahiltna assemblage (J–K) are shown inFigure 11A. Both units are part of a continuous stratigraphic pack-age that has been incorporated into two northwest-verging thrustsheets (Figs. 11B, 12A). These thrust sheets locally place theHonolulu Pass formation over the Kahiltna assemblage in the

footwalls (Figs. 11A, 12B). The Kahiltna assemblage (J–K) has adisconformable contact with the underlying Honolulu Pass for-mation (Tr–J) in the hanging walls of the thrust sheets (Figs. 11B,12A). The Caribou Pass formation occurs in the northeastern partof the study area, where it has been documented in a northeast-southwest trending syncline (K on Figs. 11A, 12A). In summary,our revised structural interpretation is that of more localized thrust-fault imbrication of the Honolulu Pass formation and Kahiltnaassemblage in contrast to previous interpretations of nappe em-placement or large-scale terrane translation.

PROVENANCE

Compositional Data

Compositional trends were determined for sandstone samplescollected within the context of measured stratigraphic sectionsfrom the Kahiltna assemblage and the overlying Caribou Pass for-mation. Standard petrographic thin sections for each sample werecut and stained for plagioclase and potassium feldspar. A total of44 thin sections were analyzed using a modified Gazzi-Dickinsonmethod of point counting (Dickinson, 1970; Ingersoll et al., 1984).Modal composition was determined by the identification of at least400 framework grains in each thin section. Point-count parametersare shown in Table 1; raw point–count data are available in GSAData Repository, Appendix 11, and recalculated data are availablein Table 2. Recalculated data are based on procedures defined byIngersoll et al. (1984) and Dickinson (1985).

Kahiltna AssemblageThe modal composition of the Kahiltna assemblage is char-

acterized by predominantly quartz and roughly equal amounts offeldspar and lithic grains (Q-43% F-30% L-27%). Quartz grainsconsist of monocrystalline quartz (Qm), polycrystalline quartz(Qp), and chert (C; Figs. 13A–13D) with polycrystalline quartzbeing the most common constituent. The feldspar population inthese sandstones is dominated by plagioclase grains (P; Qm-37%P-61% K-2%; Fig. 13D). Lithic grains consist largely of volcanictypes (Lv; Figs. 13B, 13C) with metamorphic (Lm) and sedimen-tary types (Ls) being less common (Lv-71% Lm-18% Ls-11%).Common textures observed in lithic volcanic grains include pilo-taxitic microlitic (Fig. 13B), lathwork (Figs. 13B, 13C), vitric,and felsitic. Lithic metamorphic grains are mainly micaceousschist and fine-grained pelitic schist. Sedimentary lithic grains(Ls) consist primarily of siltstone and mudstone.

Caribou Pass FormationThe Caribou Pass formation is made up predominantly of

quartz grains (largely polycrystalline) with subordinate lithic frag-ments and feldspar grains (Q-65% F-9% L-26%; Figs. 13E–13H).

1GSA Data Repository Item 2007111, Appendices 1 and 2, is available on requestfrom Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA,editing@geosociety.org, or at www.geosociety.org/pubs/ft2007.htm.

TABLE 2. RECALCULATED MODAL POINT–COUNT DATA FOR SANDSTONE SAMPLES OF THE KAHILTNA ASSEMBLAGE AND CARIBOU PASS FORMATION

Q-F-L % Qm-F-Lt % Qm-P-K % Qp-Lvm-Lsm % Lv-Lm-Ls %

Sample No. Q F L Qm F Lt Qm P K Qp Lvm Lsm Lv Lm Ls

Caribou Pass formation n � 26 FLR2-9mFLR2-19mFLR2-42mFLR2-60mFLR2-70mFLR2-96mFLR2-156mFLR2-163mFLR-072902-01FLR2-072902-01FLR2-072902-02FLR-072902-03FLR-072902-03 (a)FLR-072902-03 (b)FLR-072902-07FLC-072102-01FLC-072102-02FLR-072202-01FLR-12mFLR-18mFLR-46mFLR-66mFLR-102mFLR-113mFLRfl (a)FLRfl (b)

Caribou Pass formation n � 26 TCB 14mTCB 39mTCB 39.5mTCB 94mTCB 205mTCB 232mTCB 071302-1EFC 071102-03EFC 071102-04EFC 071502-01(a)EFC 071502-01(b)EFC 071502-01(c)EFC 071502-01(d)EFC 071502-01(e)AC 072002-01AC 072002-02AC 072002-03AC 072002-04

6072746264656461717581495153667480757768656961686868

494857494946492139444644454137384127

16949

171271

1157

162019544478554957

272317272633314630262928342433343745

2419222919232938182012352928292216211624302635232725

242926242521203331302528213530282228

122019116

13107

1614218

121712102624251418188

151516

101316161921318

16222418221517172410

16949

171371

1067

162019544477554858

272317272634314630262929342532343845

7271778077748392748072766864838670726879777788778076

636467575545384654524753446151493845

4368835528526090607276343847697487867865777963647569

283747374139491536464539393834333819

4428174472473910392824666253312613142235232137362531

696353595443508364525561615965676281

134010110100000000000000000

30045

18120200031000

6774716475696558767683535757657477717669626660706769

615462575554472941414748524441424437

201412131162

25673

291621562338

109

22132014

333931313835396354524734374349544661

1312172314253317181714182722302021262123282518171317

677

127

11148576

181113124

102

5953402537166

502027165425421166

111325171739334633

725667688074698182728546637265876383

3240587054688941735563274930597063534538556646403935

2335211213148

1115227

342414228

2414

97259

16597

1821192628302431364237281715271532

59

12207

12238368

201314135

133

422 Hampton et al.

Polycrystalline quartz (Qp; Figs. 13E, 13F, 13G), lithic metamor-phic grains (Lm) of pelitic schist, and mica schist are common(Fig. 13G), with quartzite (Fig. 13H) being a lesser constituent.Plagioclase feldspar (P) is more common than potassium feldspar(Qm-65% P-34% K-1%), and metamorphic types (Lm) are themost common lithic grain (Lv-25% Lm-51% Ls-24%). Lithic vol-canic (Lv) grains commonly exhibit felsitic textures in addition tomicrolitic and lathwork textures (Fig. 13F, 13H). Sedimentarygrains (Ls) consist primarily of siltstone and sandstone and makeup nearly a quarter of the total lithic fragments.

Summary: Compositional Trends and Potential Sources

Our compositional data from the Kahiltna assemblage andCaribou Pass formation document some significant differencesbetween the two units (Fig. 14). There is a noticeable difference,for example, in the relative abundance of quartz and feldsparbetween the units (Kahiltna assemblage: Q-43% F-30% L-27%;Caribou Pass formation: Q-65% F-9% L-26%). The Kahiltnaassemblage also contains a relative abundance of lithic volcanicgrains compared to the Caribou Pass formation (Fig. 14). TheCaribou Pass formation, in contrast, is enriched in polycrystallinequartz and lithic metamorphic and sedimentary grains relative tothe Kahiltna assemblage (Fig. 14). These compositional differ-ences suggest sediment derivation from distinct source terranesfor each unit. The composition of the Kahiltna assemblagerequires it to have been derived from a source rich in volcanicrocks. The composition of the Caribou Pass formation, in con-trast, requires derivation from a source terrane consisting ofquartz-rich metamorphic and sedimentary rocks. Comparing ourcompositional data with the provenance fields of Dickinson et al.(1983), the Kahiltna assemblage plots with sandstones derivedfrom arc sources (transitional to dissected) with minor contribu-tions from recycled orogen sources, whereas the Caribou Passformation plots with sandstones derived primarily from recycledorogen sources (Figs. 14A, 14B).

The Kahiltna assemblage and Caribou Pass formation in thestudy area are located in the Mesozoic suture zone betweenoceanic rocks of the Wrangellia composite terrane and metamor-phosed continental-margin rocks of the Yukon-Tanana terrane(Fig. 1A; e.g., Ridgway et al., 2002). Our compositional data sug-gest that both of these terranes provided detritus for Mesozoicsandstones currently exposed in the northwestern TalkeetnaMountains. Volcanic detritus was likely derived from more arc-rich parts of the Wrangellia composite terrane (Wrangellia andPeninsular terranes) during the Late Jurassic–Early Cretaceous(Kimmeridgian–Aptian) deposition of the Kahiltna assemblage.We do not rule out the possibility that Paleozoic arc rocks of theYukon-Tanana terrane contributed some detritus during this time.In fact, our detrital zircon data discussed in the next section sug-gest that the Yukon-Tanana terrane contributed sediment duringdeposition of the Kahiltna assemblage. By the end of Early Cre-taceous time (Albian/Cenomanian), however, quartz-rich meta-

morphic rocks of the Yukon-Tanana terrane supplied the bulk ofthe sediment during deposition of the Caribou Pass formation. Insummary, the sandstone compositional data indicate that therewas a fundamental shift from arc-derived sediment from sourceslocated south of the study area to continental margin-derived sedi-ment from sources located north and east of the study area duringAlbian time.

U-Pb Detrital Zircon Analysis

MethodologyZircons were collected from four sandstone samples of the

Kahiltna assemblage in the northwestern Talkeetna Mountains.(See Fig. 1B for geographic location of the samples and Fig. 7 forstratigraphic location of samples within the context of our mea-sured sections.) These samples were analyzed with a MicromassIsoprobe multicollector Inductively Coupled Plasma Mass Spec-trometer (ICPMS) equipped with nine Faraday collectors, an axialDaly detector, and four ion-counting channels. The Isoprobe isequipped with a New Wave DUV 193 laser ablation system withan emission wavelength of 193 nm. The analyses were conductedon 35–50 micron spots with an output energy of �40 mJ and arepetition rate of 8 hz. Each analysis consisted of one 20-secondintegration on the backgrounds (on peaks with no laser firing) andtwenty 1-second integrations on peaks with the laser firing. Thedepth of each ablation pit is �20 microns. The collector configu-ration allows simultaneous measurement of 204Pb in a secondaryelectron multiplier, whereas 206Pb, 207Pb, 208Pb, 232Th, and 238U aremeasured with Faraday detectors. All analyses were conducted instatic mode.

Correction for common Pb was done by measuring 206Pb/204Pb,with the composition of common Pb from Stacey and Kramers(1975) and uncertainties of 1.0 for 206Pb/204Pb and 0.3 for 207Pb/204Pb. Fractionation of 206Pb/238U and 206Pb/207Pb during ablationwas monitored by analyzing fragments of a large concordant zir-con crystal that has a known isotope dissolution-thermal ioniza-tion mass spectrometry (ID-TIMS) age of 564 � 4 Ma (2-sigma;George Gehrels, 2006, personal commun.). Typically this refer-ence zircon was analyzed once for every four unknowns. Theuncertainty arising from this calibration correction, combinedwith the uncertainty from decay constants and common Pb com-position, contributes �1% systematic error to the 206Pb/238U and206Pb/207Pb ages (2-sigma level). The preferred ages are based on206Pb/238U ratios for <1.0 Ga grains and on 206Pb/207Pb for >1.0 Gagrains. Asummary of measured isotopic ratios and ages is reportedin GSA Data Repository, Appendix 2 (see footnote 1). For eachsample, the analyses are shown on a Pb/U concordia diagram(Fig. 15A) and on an age probability plot (Fig. 16; using plottingprogram of Ludwig, 2003). The latter is generated by summingthe individual probability distributions for all grains in a sample.Also shown is a plot of U/Th versus age (Fig. 15B). Uranium andthorium concentrations are calibrated by analysis of NBS SRM610 trace element glass.

AKahiltna assemblage

B

C

D

E

F

G

H

QmLv

C

C

C

C

C

C

CC

C C

C

C

CC

Qm

Qm

Qp

Qp

Qp

QpQp

QpQp

Qp Qp

Qm Qm

Qm

Calcite

Qp

QpQp

Qp

Qp

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Qp

Qp

C

C

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Qm

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Qm

Qm Qm

Qm

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P

Lv

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Lm

PQm

Qm

QmQm

Lm

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C

Lv

Lv

Lv

Lv Lv

100100 um

250250 um

100100 um

100100 um 100100 um

100100 um

100100 um

250250 um

Calcite

Caribou Pass formation

Figure 13. Photomicrographs of the Kahiltna assemblage and overlying Caribou Pass formation (note scale bar in the lower right corner of eachphoto). Kahiltna assemblage. (A) Monocrystalline quartz (Qm) and plagioclase (P) together with lithic volcanic grains are the main constituentsof the Kahiltna assemblage. (B) Grains of monocrystalline quartz (Qm), lithic volcanics (Lv), plagioclase (P), and chert (C) with subordinatepolycrystalline quartz (Qp). (C) Outsized lithic volcanic grain (Lv) surrounded by monocrystalline quartz (Qm), polycrystalline quartz (Qp),plagioclase (P), and chert (C). (D) Plagioclase (P), chert (C), monocrystalline quartz (Qm), and polycrystalline quartz (Qp) are common insandstone of the Kahiltna assemblage. Caribou Pass formation. (E) Polycrystalline quartz (Qp) is one of the primary constituents of the Cari-bou Pass formation. (F) Polycrystalline quartz (Qp), lithic volcanic grain (Lv), monocrystalline quartz (Qm), and chert (C). (G) An elongatequartz-rich lithic metamorphic grain (Lm), polycrystalline quartz (Qp), chert (C), and monocrystalline quartz (Qm). (H) Lithic metamorphicgrains (Lm) of quartzite (larger Lm grain) and schist (smaller Lm grain), lithic volcanic grain (Lv), monocrystalline quartz (Qm), chert (C),and plagioclase (P).

424 Hampton et al.

U-Pb Age DistributionThe Kahiltna assemblage of the northwestern Talkeetna Moun-

tains shows numerous occurrences of concordant Precambrian–Mesozoic age zircons (Fig. 15) with Mesozoic (Mz) age grainsbeing most abundant, less-abundant middle and upper Paleozoic(Pz) grains, and limited occurrence of Precambrian (Pc) age grains(Mz-84% Pz-11% Pc-5%; Fig. 16). The majority of Mesozoic agegrains have ages between 240 and 160 Ma (peak occurrence be-tween 205 and 195 Ma) and 130–110 Ma (peak occurrence from

125 to 115 Ma). Paleozoic age grains occur within a window of380–300 Ma with one sample having a few additional occurrencesbetween 300 and 280 Ma and 420–380 Ma (sample TCB-205 onFig. 16). The majority of Precambrian age grains fall between 1.9and 1.6 Ga (�56% of total Pc grains) and 3.0–2.0 Ga (�28% oftotal Pc grains) with the remainder of isolated grains near 1.4 Ga(5% of total Pc grains) and around 620 Ma (11% of total Pcgrains).

U-Pb Maximum Depositional AgesThe youngest and oldest concordant ages from these samples

are 106 � 7 Ma and 2695 � 12 Ma, respectively. Because theyoungest age may have experienced Pb loss, a more reliable min-imum crystallization age (maximum depositional age) is deter-mined from the youngest cluster of three or more overlapping andconcordant analyses. The most likely ages of these clusters can bedetermined from peaks in the age probability plots (Fig. 16) orfrom the TuffZirc program of Ludwig (2003). For each samplethese analyses yield maximum depositional ages as follows: sam-ple EFC-071102–04 peak age � 117.6 Ma, and TuffZirc age �116.1 �2.8/�2.7 Ma; sample HPC-072802–02 peak age �124.3 Ma, and TuffZirc age � 124.1 �5.0/�4.4 Ma; sampleTCB-205 peak age � 126.7 Ma, and TuffZirc age � 125.4 �4.8/�4.9 Ma; sample AC-072002-01 peak age � 124.5 Ma, andTuffZirc age � 127.0 �3.3/�7.6 Ma (all uncertainties are at 2-sigma and include all random and systematic errors). If theyoungest 83 analyses from these samples are grouped together,the peak in age probability occurs at 119.2 Ma and TuffZirc iden-tifies 48 analyses that cluster at 118.3 �1.4/�2.4 Ma. All of theseages are within the Aptian according to the time scale of Grad-stein et al. (2004). It is important to note that there are a numberof individual grains that are as young as Albian. The revised Apt-ian age presented here for the Kahiltna assemblage in the north-western Talkeetna Mountains is determined from peak ages andconsidered to be a robust maximum depositional age.

Previous studies have cited the age of the Kahiltna assemblagein this area as Upper Jurassic–Lower Cretaceous (Kimmeridgian–Valanginian) based on Buchia macrofossils (Jones et al., 1980;Silberling et al., 1981a; Smith et al., 1988). We suggest that theupper age of the Kahiltna assemblage may be at least 10–15 m.y.younger than previously reported based on maximum deposi-tional ages from the detrital zircon data. The new age range forthe Kahiltna assemblage employing both biostratigraphy and detri-tal zircon age data is Kimmeridgian–Aptian.

Provenance of Detrital ZirconsMatching U-Pb ages of detrital zircons from the Kahiltna

assemblage with U-Pb ages of possible source terranes providesa powerful provenance tool. The low ratios of uranium-thoriumrecorded from detrital zircon grains within the Kahiltna assem-blage (values <8; Fig. 15B) suggest that these grains were derivedfrom igneous rather than metamorphic source areas. Figure 17provides a general summary of possible igneous source areaslocated north and south of the study area.

Q

F L

Dissected arc

Transitional arc

Undissected arc

Continental block- basement uplift Recycled

orogen

Qm

F Lt

Mixed Recycled orogen

Continental block- basement uplift

Mean

N = 44

Qm

P K

Qp

Lvm Lsm

Lv

Lm Ls

Kahiltna assemblage

Caribou Pass formation

Figure 14. Ternary diagrams showing the modal composition of sand-stone from the Kahiltna assemblage and the Caribou Pass formation. Theblack circles depict the mean modal composition, and the gray and whiteareas within polygons represent one standard deviation from the mean.See Table 1 for explanation of abbreviations. Note the relative increasein quartz and decrease in feldspar between the sandstone of the Kahiltnaassemblage and that of the overlying Caribou Pass formation. Also notethe relative increase in Qp and Lm in the Caribou Pass formation relativeto the Kahiltna assemblage. This difference in composition suggests thatthe Caribou Pass formation received a larger amount of detritus from aquartz-rich metamorphic source terrane than the Kahiltna assemblage,which is enriched in plagioclase and lithic volcanic grains. Provenancefields of Dickinson et al. (1983) suggest a mixed-arc source area withminor contributions from recycled orogen sources for the Kahiltnaassemblage and a predominantly recycled orogen source for the CaribouPass formation.

2600

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1800

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0.2

0.3

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207Pb/235U

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AC-072002-01 (n=93) TCB-205 (n=80)

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a)

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EFC-071102-04 (n=99)

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2200U/Th versus age plot for all grains (n=369)

A

B

Figure 15. (A) U-Pb concordia diagrams of single detrital zircon grains from four samples of sandstone from the Kahiltna assemblage. Enlarge-ments on right of each plot show data for grains <542 Ma. Concordia plots were produced using the programs of Ludwig (1991a, 1991b). Allerrors are shown at the 95% confidence level. Sample number and number of grains counted (n) are shown in box in the upper left corner ofeach plot. See Figure 7 for stratigraphic position of samples within our measured sections. (B) Plot of U/Th versus age for all samples (dashedline represents U/Th value of 5). Note that <1.5% of total samples have U/Th values >5.

426 Hampton et al.

TOTAL n = 369

Mesozoic ages n = 309Paleozoic ages n = 42Precambrian ages n = 18

RE

LA

TIV

E F

RE

QU

EN

CY

AGE (Ma)

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20

040 80 120 160 200 240 280 320 360 400 440 480 520

Cenozoic Mesozoic PaleozoicHPC-072802-02 (n=97) maximum depositional age (MDA) 124.3 Ma

Phanerozoic ages n=93

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Cenozoic Mesozoic PaleozoicEFC-071102-04 (n=99) maximum depositional age (MDA) 117.6 Ma

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Cenozoic Mesozoic PaleozoicAC-072002-01 (n=93) maximum depositional age (MDA) 124.5 Ma

Phanerozoic ages n=9010

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Cenozoic Mesozoic Paleozoic TCB-205 (n=80) maximum depositional age (MDA) 126.7 Ma

Phanerozoic ages n=7410

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MDA

Figure 16. Age probability histogram plotsshowing the distribution of detrital zirconages from four sandstone samples of theKahiltna assemblage. Black histogramsare in 20 m.y. intervals, and gray his-tograms are in 5 m.y. intervals. The totalnumber of grains analyzed was 369. Thepercentage of Mesozoic (Mz), Paleozoic(Pz), and Precambrian (Pc) grains fromthese four samples is Mz-84% Pz-11%Pc-5%. Note the abundance of grains withages that fall between 130 and 110 Ma(peak occurrences from 125 to 115 Ma)and between 240 and 160 Ma (peak occur-rences between 205 and 195 Ma). Themajority of Paleozoic age grains fall be-tween 380 and 300 Ma. Most Precambriangrains have ages between 2100 and 1700Ma. MDA stands for maximum deposi-tional age (determined from age probabil-ity plots where peaks are determined fromthe youngest cluster of three or more over-lapping and concordant analyses).

It is important to note that Cretaceous–Tertiary dextral dis-placement (up to 400 km) along the Denali fault is thought to havebeen largely responsible for much of the geology observed insouthern Alaska and western Canada (e.g., Eisbacher 1976; Nokle-berg et al., 1985; Plafker and Berg, 1994; Cole et al., 1999; Tropet al., 2004; Trop and Ridgway, this volume). Given the tectoniccomplexity of the North American Cordillera since the middle ofthe Mesozoic, an investigation into potential source areas fordetrital zircon grains should ideally involve a comprehensiveexamination of the entire Cordillera. Such an approach is beyondthe scope of this study given the limited number of samples. Wedo, however, provide a general summary of potential igneoussources in southern Alaska and the adjacent Yukon Territory.

Potential sources south of study area. Some of the morewidespread potential Mesozoic igneous source areas located

south of the study area include (1) Upper Triassic–Jurassic rocksof the Peninsular terrane (including the Talkeetna arc) and partiallycoeval Jurassic granitoid rocks of the Chitina arc in the WrangellMountains (Fig. 17), and (2) Cretaceous granitoid rocks of theChisana arc in the Wrangellia terrane (Fig. 17). Igneous activityassociated with the Chisana arc ranges from ca. 140–125 Ma;however, new 40Ar/39Ar ages from volcanic rocks (113 � 1.3 Maand 116 � 1.3 Ma) and cooling ages from granitoid rocks (113 �1.3 Ma and 117 � 0.54 Ma) suggest that the arc was active at leastuntil Late Cretaceous time (Short et al., 2005; Snyder and Hart,2005, this volume).

Granitoid rocks associated with the Peninsular terrane (Tal-keetna arc and equivalent rocks) are exposed north of the BorderRanges fault and south of the Kahiltna assemblage in south-centraland southwestern Alaska (Fig. 17). The oldest ages reported for

J-K

J-KPz-Tr

0 km 100 km

DF

DF

TF

TF

Skolai arc (320 285 Ma)

Chitina arc (175 135 Ma)Talkeetna arc/equiv. Border Ranges mafic/ultramafic complex

Chisana arc (140 115 Ma)

(201 153 Ma)

Taylor Mtns. batholith (215 175 Ma)

SOUTH OF STUDY AREA - Wrangellia composite terrane

Possible plutonic sources Age range

Peninsular terrane (P) - Tr J sedimentary and igneous rocks

Wrangellia terrane (W) - Penn Tr sedimentary and igneous rocks

Undifferentiated plutonic suites and (380 330 Ma)metasedimentary assemblages of the Yukon-Tanana terrane

Undifferentiated Cretaceous plutons (110 85 Ma)

NORTH/EAST OF STUDY AREA - Yukon-Tanana terrane

Possible plutonic sources Age range

Yukon-Tanana terrane - (YTT)

Kahiltna assemblage/equivalent - (J K)

Undifferentiated rocks

WW

- Hines Creek fault- Strike-slip fault

- Fault (undifferentiated)

- Border Ranges fault

- Denali fault

HCF

FAULT LEGEND

DF

- Talkeetna fault TF

BRF

HCF

Prince William Sound

STUDY AREA

Source areas

Cook Inlet

Anchorage

Pacific Ocean

FairbanksNAlaska

150º 145º 140º

155º

65º

60º

BRF

DF

Alexander terrane (A) - Camb Tr. metasedimentary and igneous rocks

A

FIGURE KEY

Figure 17. A simplified geologic map of southern Alaska and adjacent Yukon Territory that shows the distribution of the Kahiltna assemblage(J–K), as well as potential igneous source areas that may have provided sediment to the Kahiltna assemblage. Potential source areas includeplutonic rocks of the Wrangellia composite terrane located south and east of the outcrop belt of the Kahiltna assemblage and the Yukon-Tananaterrane located to the north and northeast of the outcrop belt. Specific ages of arc/plutonic rocks that have some overlap with the ages of detri-tal zircons grains from the Kahiltna assemblage are shown in the legend. See text for additional discussion.

428 Hampton et al.

the Talkeetna arc are between 201 and 198 Ma (Pálfy et al., 1999;Rioux et al., 2005; Amato et al., this volume, chapter 11). Meso-zoic magmatism associated with the Talkeetna arc is thought tohave taken place from 201–153 Ma with punctuated episodes ofplutonism between 201 and 181 Ma and between 177 and 153 Ma(Onstott et al., 1989; Rioux et al., 2002, 2003, 2005). Ultramaficrocks, informally referred to as the Border Ranges ultramafic-mafic complex (Burns, 1985) are thought to be slightly olderto contemporaneous with intermediate plutons and have beenproposed to represent the deepest part of the Talkeetna arc(Fig. 17; Burns, 1985). It should be noted that, although gener-ally thought to be contemporaneous with the Talkeetna arc, adiorite crystallization age as old as 217 � 10 Ma has been re-ported from the Border Ranges mafic-ultramafic complex (Roeskeet al., 1989). Upper Jurassic plutonic and metaplutonic rocksof the Chitina arc are exposed southeast of the study area inthe Wrangell Mountains and in the Yukon Territory (Fig. 17)and represent igneous activity from ca. 175–135 Ma (Plafkeret al., 1989; Nokleberg et al., 1994; Roeske et al., 2003). TheChitina arc intruded the Wrangellia terrane and was partiallycoeval with the Talkeetna arc of the Peninsular terrane (Fig. 17;Trop et al., 2005).

Although the Kahiltna assemblage contains Triassic (majoritybetween ca. 240–200 Ma) and mid-Paleozoic (majority betweenca. 380 and 300 Ma) age detrital zircons (Fig. 16) there are fewknown source areas south of the study area that would contributegrains of these ages. It should be noted that Paleozoic igneousrocks associated with the Skolai arc are rare but do occur south ofthe study area in parts of the Wrangellia and the Alexander ter-ranes in southeastern Alaska (Fig. 17). The range of ages for theserocks is around 320–285 Ma with the majority of ages around 310 Ma (Nokleberg et al., 1986; Aleinikoff et al., 1988; Gardneret al., 1988; Beard and Barker, 1989; Plafker et al., 1989); these agesare slightly younger than the Devonian–Mississippian detrital peaksobserved in the Kahiltna assemblage (Fig. 18). The Alexander ter-rane in southeastern Alaska contains Permian–Triassic (ca. 280–220 Ma) and Ordovician–Silurian (480–410 Ma) plutonic suites(Gehrels and Saleeby, 1987; Gehrels, 1990) that are likely sourcesfor Triassic grains older than ca. 215 Ma, as well as the Permianzircons documented in the Kahiltna assemblage.

Potential sources north and east of the study area. Some ofthe oldest Mesozoic plutonic rocks in Alaska are found north ofthe Denali fault and consist of Upper Triassic to Lower Jurassicplutons that make up the Taylor Mountains batholith of the Yukon-Tanana terrane in east-central Alaska (Fig. 17). A host of stud-ies have presented U-Pb, 40Ar/39Ar, and K-Ar ages from UpperTriassic–Lower Jurassic plutons from this region that fall be-tween 215 and 175 Ma (e.g., Aleinikoff et al., 1981; Cushing,1984; Wilson et al., 1985; Foster et al., 1994; Dusel-Bacon et al.,2002). Younger plutons, Lower to Upper Cretaceous age, havebeen reported from segments of the Yukon-Tanana terrane withages ranging from 110–85 Ma (Foster et al., 1994). These plutonswere not a major source but likely contributed some of theyoungest grains reported from the Kahiltna assemblage.

Devonian and Mississippian age plutonic rocks from seg-ments of the Yukon-Tanana terrane (Fig. 17) consist of augengneiss, dioritic orthogneiss, and granitic orthogneiss, all of whichhave originated from plutonic protoliths that were part of a middlePaleozoic continental magmatic arc (Dusel-Bacon and Aleini-koff, 1985). U-Pb zircon SHRIMP ages, U-Pb concordia ages, andRb-Sr whole-rock isochron ages from these Paleozoic plutonicrocks have an age range between ca. 380 and 330 Ma (Mortensen,1983; Aleinikoff et al., 1986; Dusel-Bacon and Aleinikoff, 1985;Dusel-Bacon et al., 2001; Day et al., 2003).

Although Precambrian age detrital zircons make up a smallpercentage of the grain population in the Kahiltna assemblage(�5%), it is important to consider possible source areas becausetheir occurrence suggests that this region was in proximity to thecontinental margin and receiving detritus from North Americaduring Cretaceous time. Precambrian basement provinces in west-ern North America consist of the Canadian Shield (>2.5 Ga) andsurrounding cratonic rocks in western Canada that include agesfrom 2.4–2.0 Ga and from 2.0–1.8 Ga. In the western U.S., theprimary Precambrian source areas include rocks of Grenville age(1.2–1.0 Ga) and Yavapai-Mazatzal age (1.87–1.67 Ga). Age rangesfor Precambrian basement are summarized from Hoffman (1989),Ross (1991), and Reed (1993). Sedimentary rocks of the Belt-Purcell and Windermere Supergroup in western Alberta, Mon-tana, and Idaho are a potential source of recycled detrital zirconsand contain a range of Precambrian age detritus from the westernUnited States and western Canada (Ross et al., 1992). Occur-rences of Precambrian grains ranging from 3.0–1.0 Ga have alsobeen reported from the Alexander terrane (Gehrels et al., 1996).

Interpretation of U-Pb Detrital Zircon DataFigure 18 outlines our interpretation for the provenance of

Mesozoic and Paleozoic detrital zircons documented in theKahiltna assemblage of the northwestern Talkeetna Mountains.Mesozoic detrital zircon grains make up �84% of the total grainsfrom the Kahiltna assemblage. Approximately 19% of the detri-tal zircon grains from the Kahiltna assemblages have Mesozoicages younger than 140 Ma (Fig. 18). Of these grains, ages that fallbetween 140 and 115 Ma (�15% of total grains) were most likelyderived from granitoid rocks of the Chisana arc of Wrangellialocated southeast of the study area (Fig. 17). The remaining �4%of Mesozoic grains are younger than 115 Ma and were likelyderived from Cretaceous plutons of the Yukon-Tanana terranelocated northeast of the study area.

The bulk of the Mesozoic detrital zircons have age rangesbetween 215 and 135 Ma (�55% of total grains; Fig. 18). Weinterpret these grains to have been derived in part from the Tal-keetna-Chitina arc (south of the study area) and the Taylor Moun-tains batholith (northeast of the study area). It is important to notethe overlap in ages between the Chitina arc (175–135 Ma) andTalkeetna arc (201–153 Ma), as well as the Talkeetna arc and theTaylor Mountains batholith (215–175 Ma). Based on these over-lapping ages, Mesozoic grains with ages between 153 and 135 Ma(�2% of total grains) were likely derived solely from plutons of

RE

LA

TIV

E F

RE

QU

EN

CY

AGE (Ma)

Yukon-Tanana terrane Late Cretaceous plutons (85 110 Ma)

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040 80 120 160 200 240 280 320 360 400 440 480 520

Cenozoic Mesozoic PaleozoicHPC-072802-02 (n=97) maximum depositional age (MDA) 124.3 Ma

Wrangellia Chisana arc(115 140 Ma)

Wrangellia-Peninsular terranes Chitina arc - (135 170 Ma) Talkeetna arc - (153 201 Ma)

Yukon-Tanana terraneTaylor Mtns. batholith (175 215 Ma)

Alexander terrane Perm. Triassic plutons (ca. 220 280 Ma)

Wrangellia Skolai arc(285 320 Ma)

Yukon-Tanana terrane Dev. Miss. plutons (330 380 Ma)

Cenozoic Mesozoic PaleozoicEFC-071102-04 (n=99) maximum depositional age (MDA) 117.6 Ma

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Cenozoic Mesozoic PaleozoicTCB-205 (n=80) maximum depositional age (MDA) 126.7 Ma

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Cenozoic Mesozoic PaleozoicAC-072002-01 (n=93) maximum depositional age (MDA) 124.5 Ma

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Alexander terrane Sil. Ord. plutons (410 480 Ma)

Phanerozoic ages n=93

Phanerozoic ages n=90

Phanerozoic ages n=74

Phanerozoic ages n=95

Figure 18. Age probability histogram plotsshowing the distribution of Mesozoic andPaleozoic detrital zircons ages from foursandstone samples of the Kahiltna assem-blage. Black histograms are in 5 m.y. in-tervals. The age range for Mesozoic andPaleozoic potential source areas from theWrangellia composite terrane and Yukon-Tanana terrane are shown to compare agedistribution of detrital grains with knownages of plutonic arc suites adjacent to thestudy area. See Figure 17 for geographiclocations of potential source areas. Themain sources of detritus during deposi-tion of the Kahiltna assemblage appear tobe the Chisana arc, Talkeetna-Chitina arcs,and Permian–Triassic plutons (Alexanderterrane) of the Wrangellia composite ter-rane and the Taylor Mountains batholithof the Yukon-Tanana terrane. Devonian–Mississippian age zircon grains of theKahiltna assemblage were also likelyderived from the Yukon-Tanana terrane.The youngest and oldest zircon grains werelikely derived from the Yukon-Tanana ter-rane and Alexander Terrane, respectively.See text for additional discussion.

430 Hampton et al.

the Chitina arc. Detrital zircons with age ranges between 201 and153 Ma (�37% of total grains) are interpreted to represent deriva-tion from a combination of the Chitina and Talkeetna arcs as wellas the Taylor Mountains batholith. Mesozoic ages between 215and 201 Ma (�16% of total grains) were likely derived solelyfrom the Taylor Mountains batholith of the Yukon-Tanana terrane.The remainder of Mesozoic detrital zircon grains documented inthe Kahiltna assemblage that are older than 215 Ma (�10% oftotal grains) were most likely derived from younger parts ofPermian–Triassic plutons of the Alexander terrane (Fig. 18).

Paleozoic age detrital zircon grains make up �11% of thetotal grains from the Kahiltna assemblage and are interpreted tohave been derived from source areas related to the Skolai arc(320–285 Ma); undifferentiated augen gneiss, dioritic orthogneiss,and granitic orthogneiss from segments of the Yukon-Tanana ter-rane (380–330 Ma); and a possible contribution from Ordovician–Silurian plutonic rocks of the Alexander terrane (480–410 Ma;Figs. 17, 18). Of these potential sources, �14% of Paleozoic agedetrital zircon grains from the Kahiltna assemblage (<2% of totalgrains) fall within the range of the Skolai arc and �55% (�6% oftotal grains) are within the range of igneous sources of the Yukon-Tanana terrane. Approximately 10% are split between age rangesfor Permian–Triassic plutons (5%) and Ordovician–Silurian plu-tons (5%) of the Alexander terrane (Fig. 18). The remaining �21%of Paleozoic age detrital zircon grains are between 330 and 320 Ma(�12%) and between 410 and 380 Ma (�9%). These age rangesfall between the oldest ages of the Skolai arc and the youngestages of Mississippian plutonic rocks of the Yukon-Tanana terraneand between the oldest ages of Devonian plutonic rocks of theYukon-Tanana terrane and the youngest ages of Silurian plutonicrocks of the Alexander terrane, respectively (Fig. 18).

Precambrian grains make up �5% of the total zircon popu-lation from the Kahiltna assemblage. Up to �56% of Precam-brian detrital zircon grains have ages between 1.9 and 1.6 Ga and�28% are between 3.0 and 2.0 Ga (Fig. 15). The remainder ofages for the Precambrian grains are isolated near 1.4 Ga (5% oftotal Pc grains) and ca. 620 Ma (11% of total Pc grains). Ages >2.0Ga and between 1.9 and 1.6 Ga were likely derived from north-ern locations within the North American Cordillera (north of lati-tude 40°N) that include the Canadian Shield and recycled detritalzircons from the Windermere Supergroup and Belt-Purcell Group(e.g., Gehrels et al., 1995; Mahoney et al., 1999). Younger Pre-cambrian detrital zircons likely originated south of latitude 40°Nand may have come to their present location by multiple recyclingevents throughout the evolution of the Cordillera (e.g., Ross et al.,1992; Gehrels et al., 1995). Precambrian grains ranging from 3.0to 1.0 Ga have also been reported from the Alexander terrane(Gehrels et al., 1996). Due to the small number of Precambriangrains in the Kahiltna assemblage, we do not use the occurrenceof Precambrian detrital grains to infer amount of translation ortransport along the western North American continental margin;rather, we use these occurrences to strengthen our contention that the Kahiltna basin was receiving detritus from continental-margin sources during Early Cretaceous time.

In summary, our U-Pb detrital zircon data suggest that theUpper Jurassic–Lower Cretaceous Kahiltna assemblage in thestudy area was receiving sediment from both arc-related rocks ofthe Wrangellia composite terrane to the south and continental-margin rocks of the Yukon-Tanana terrane to the northeast. Thebulk of the zircon ages can be attributed to theWrangellia compos-ite terrane, but there is a clear signal of sediment contribution fromcontinental margin sources (minimum of �26% of total grains).

DISCUSSION

Regional Stratigraphic Correlation Along the Suture Zone

Upper Triassic–Cretaceous strata located between the Wran-gellia composite terrane and the Mesozoic continental margin ofNorth America are exposed in an elongate, southwest-trendingbelt that has been studied mainly in the context of regional map-ping projects (Fig. 19). Figure 20 presents a summary of our tenta-tive stratigraphic correlations of Upper Triassic–Cretaceous strataalong the suture zone. In the following text, we briefly discussthese potential stratigraphic relationships. Our regional strati-graphic summary does not necessarily imply that all Mesozoicstrata were deposited in the same basin; however, it does point tosome common stratigraphic elements in all the basinal settings.

Lake Clark RegionThe Mesozoic stratigraphy in this region of southwestern

Alaska (location 1 on Figures 19, 20) is defined by a successionthat consists of the Upper Triassic–Lower Jurassic (?) Chi-likadrotna greenstone sequence, the Upper Jurassic–Lower Cre-taceous Koksetna River sequence, and the Upper CretaceousKuskokwim Group (Eakins et al., 1978; Hanks et al., 1985; Wal-lace et al., 1989; Elder and Box, 1992). The Chilikadrotna green-stone sequence and the Koksetna River sequence have beencollectively referred to as the southern Kahiltna terrane by Wal-lace et al. (1989). The main part of the Chilikadrotna greenstonesequence consists of Upper Triassic (Norian) interbedded mas-sive lavas (locally pillow lavas), limestone, and chert. Conodontsfrom the limestone units have been assigned a Late Triassic(Norian) age (Wallace et al., 1989). The top of the Chilikadrotnagreenstone sequence is defined by andesite flows, tuffs, and tuffbreccia. No bounding ages have been recorded from the top orbottom of the sequence; thus, the upper and lower age extentremains unknown. Further study is necessary to determine if thestratigraphy at the top of the Chilikadrotna greenstone sequencerepresents a condensed stratigraphic section similar to thatdescribed for the Honolulu Pass formation (Fig. 20). The Chi-likadrotna greenstone has been tentatively correlated with thePeninsular terrane based on stratigraphic similarities (Wallace et al., 1989; Nokleberg et al., 1994).

The Chilikadrotna greenstone sequence is overlain by theUpper Jurassic–Lower Cretaceous (Kimmeridgian–Valanginian)Koksetna River sequence (Fig. 20; Bundtzen et al., 1979; Wallace

Pre-, syn-, and postcollisional stratigraphic framework and provenance of strata 431

et al., 1989). This unit consists of a volcanic lithic-rich marine suc-cession of interbedded mudstone, siltstone, fine-grained sandstone,and minor conglomerate. These strata have been interpreted as theproduct of submarine fan deposition (Wallace et al., 1989). Lim-ited sandstone compositional data from the Koksetna Riversequence indicate a predominantly arc provenance (Wallace et al.,1989); no U-Pb detrital zircon data are available from these strata.Age control for the Koksetna River sequence is based on theoccurrence of Upper Jurassic–Lower Cretaceous bivalves thatinclude Buchia mosquensis and Buchia sublaevis (Eakins et al.,1978; Wallace et al., 1989). Given a Late Triassic (Norian) age forthe Chilikadrotna greenstone sequence and the Upper Jurassic–Cretaceous (Kimmeridgian–Valanginian) age for the KoksetnaRiver sequence, there may be an extended period of nondeposi-tion represented in the stratigraphy between these two sequencesduring Early–Middle Jurassic time (Fig. 20).

Overlying the Koksetna River sequence are Upper Cretaceousstrata of the Kuskokwim Group. The contact between these twounits has not been documented in this region and may be eithera fault or stratigraphic contact. The Kuskokwim Group in theLake Clark region is thought to be Late Cretaceous (Cenomanian–

Turonian) based on the occurrence of the bivalve Inoceramus(Elder and Box, 1992). The Kuskokwim Group is interpreted tohave been deposited in a shallow marine and deltaic depositionalenvironment (Elder and Box, 1992; Miller et al., 2002).

Tatina River AreaIn the western Alaska Range, a similar stratigraphy has been

described for the Upper Triassic–Lower Jurassic Tatina River vol-canic unit, which is disconformably overlain by the Lower Creta-ceous Kahiltna assemblage (location 2 on Figs. 19, 20; Kalbaset al., this volume).TheTatina River volcanic unit disconformablyoverlies Devonian–Permian strata; all these units are consideredto be part of the Mystic subterrane (Fig. 20; Bundtzen et al., 1997).The base of the Tatina River volcanic unit consists of interbeddedpillow lavas, mudstone, chert, and volcaniclastic strata that con-tain the Triassic bivalve Monotis subcircularis. The upper partof the Tatina River volcanic unit consists of interbedded volcani-clastic sandstone, shale, and chert-pebble conglomerate and isthought to be Early Jurassic (Sinemurian) in age based on theoccurrence of ammonites and pelecypods (Reed and Nelson,1980; Bundtzen et al., 1997). Early Jurassic strata in this area are

Cook Inlet

Qal

Qal

K

K

K

J-K

J-K

Pz-Tr

Pz-Tr

Tr-J

Pz

Mz-Cz

Mz-Cz

Tr-J

0 km 100 km

Mesozoic-Cenozoic (Mz-Cz) - plutonic and volc. rocks (undifferentiated)

U Jurassic-Cretaceous (J-K) - marine sed. rocks—Kahiltna assemblage

Cretaceous (K) - nonmarine and marine sed. rocks—includes nonmarine strata of the Caribou Pass formation in study area and Kuskokwim Group to the southwest

Cret.-Cenozoic (K-Cz and Qal) - sed. and volc. rock (undifferentiated) and alluvium

Triassic- L Jurassic (Tr-J) - volc. and sed. rocks—undifferentiated terranes exposed in the northwestern Talkeetna Mtns. (includes strata of the Honolulu Pass formation in the study area)

Paleozoic (Pz) - sed. and metamorphic rocks—Yukon-Tanana terrane

Pz-Triassic (Pz-Tr) - sed./metased. and volc. rocks—undifferentiated terranes exposed north of the study area

Triassic-Jurassic (Tr-J) - volc. and sed. rocks—Peninsular terrane

Penn.-Triassic (Penn-Tr) - volc. and sed. rocks—Wrangellia island arc

Triassic-Jurassic (Tr-J) - sed. and volc. rocks—Chugach subduction complex

Thisstudy

DF

DF

BRF

BR

F

HCF

TF

- Hines Creek fault

- Strike-slip fault

- Fault (undifferentiated)

- Border Ranges fault

- Denali fault

HCF

FAULT LEGEND

DF

- Talkeetna fault TF

BRF

LITHOLOGY LEGEND

TF

Penn-Tr

Anchorage

150º

60º

155º

ChChCCCCh

WWWW

PPPP1

4

2

3

LAKE CLARK REGION Eakins et al. (1978); Wallace et al. (1989);Elder and Box, (1992)

TATINA RIVER AREA Bundtzen et al. (1997) Kalbas et al. (this volume)

COLORADO CREEK/CHULITNA REGIONJones et al. (1982); Csejtey et al. (1992); Clautice et al. (2001);Eastham et al. (2002); Ridgway et al. (2002); Trop et al. (2004)

THIS STUDYN

Figure 19. Simplified geologic map of south-central and southwestern Alaska. Note locations of previous studies within the northeast-southwesttrending belt of the Kahiltna assemblage that crops out along the northern margin of the Wrangellia composite terrane and the southern mar-gin of the Yukon-Tanana terrane. Previous studies include stratigraphic descriptions from the Upper Triassic–Cretaceous strata of the Lake Clarkregion (location 1) in southwestern Alaska (Eakins et al., 1978; Wallace et al., 1989; Elder and Box, 1992); the Tatina River area (location 2)in the southwestern Alaska Range (Bundtzen et al. 1997; Kalbas et al., this volume); and the Upper Triassic–Cretaceous strata of the ColoradoCreek/Chulitna region (location 3) in the central Alaska Range (Jones et al., 1982; Csejtey et al., 1992; Clautice et al., 2001; Eastham, 2002;Ridgway et al., 2002; Trop et al., 2004). Note the location of our study area (location 4) in the northwestern Talkeetna Mountains. See Fig-ure 20 for a general summary of the Upper Triassic–Cretaceous stratigraphy and fossil occurrences from each of these locations.

65.5

AGE (Ma)C

r e

t a

c e

o u

sJ

u r

a s

s i c

T r

i a

s s

i c

Lat

eE

arly

Lat

e

Early

Mid

dle

Lat

eE

arly

Mid

dle

Cenomanian

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

251.0

220

240

230

Norian

BathonianBajocianAalenian

Toarcian

Pliensbachian

Sinemurian

HettangianRhaetian

Callovian

Oxfordian

Cenomanian

Valanginian

Kimmeridgian

Albian

Aptian

Barremian

Hauterivian

Berriasian

Tithonian

Maastrichtian

Campanian

SantonianConiacianTuronian

C Chilikadrotna Greenstone sequence (Limestone, basalt; volc. breccia)

Koksetna River Sequence (marine)

Kuskokwim Group (marine-nonmarine)

NORTHEASTSOUTHWEST

?

?

Continuous stratigraphy

Carnian

Lower contacter co unknownnknow

?

?

?

Upper contacter co erodedroded

?

Ladinian

Anisian

Olenekian

?

?

?

m s gf

?

20 .y.2 m h~ m.yy. h0 m~20com

D~2

nfy

is20

oh

on scDis nfoonfcoDDis oD f~2~ 0 20DD

mmDisD

y. yco

honffofDiisc

~2con

20 nfo

mo

.y. h ushiat satuhiamat

tyus

orhiormmitymior ityiityi

usumatia

orhi us

tyiatmit

hiorm

(4) 126.7 Ma

(2) 124.3 Ma(1) 117.6 Ma

(3) 124.5 Ma***** Kahiltna

assemblage (marine)

Caribou Pass formation (nonmarine)

(4) NW Talkeetna Mtns. (south-central AK)

-This study

-tabular; massive

Siltstone

Mudstone

Limestone

Basalt -tabular sheets

Sandstone -lenticular; cross stratified

Sill

Sandstone/conglomerate(chert grains/pebbles)

SEDIMENTARY ROCKS

VOLCANIC ROCKS

Disconformity

* - U-Pb detrital zircon age (maximum depositional age)

- Palynomorph range - Albian-Cenomanian

*

- Leaves - Albian to younger

- Bivalve - Kimmeridgian-Valanginian-Buchia sublaevis; Buchia rugosa

-Buchia mosquensis (Lake Clark region)

- Bivalve - Early Jurassic-pectinacean or limoidean?

- Bivalve - late Norian-Monotis subcircularis

- Hydrozoan - late Norian -Heterastridium

MACROFOSSIL OCCURRENCES

- Ammonite - Sinemurian

C - Conodonts - Norian

Kahiltnaassemblage (marine)

Colorado Creek unit (marine)

Disconformityiscon

m s gf

Kahiltnaassemblage (marine)

Disconformity onfo between etwee underlying derlyMystic subterranec sub

underlain by Paleozoic strata

?

Upper contacter co erodedrode

? ?

Tatina River volcanics(Limestone,basalt; chert cong.)

m s gf

Honolulu Pass formation(Limestone, basalt;volcaniclastic; chert sandstone/cong.)

(3) Colorado Creek/Chulitna (south-central AK) Summarized from: -Eastham et al. (2002) -Ridgway et al. (2002) -Trop et al. (2004)

(1) Lake Clark region (southwestern AK) Summarized from: -Eakins et al. (1978) -Wallace et al. (1989) -Elder and Box (1992)

(2) Tatina River area (southwestern AK) Summarized from: -Bundtzen et al. (1997) -Kalbas et al. (this volume)

-gravel (g)-sandstone (s)-mudstone (m)Grain size

m s g

?

Upper contacter co erodedroded

?

Lower contacter co unknownnknow

?

?

Upper contact unknown

Lower contact unknown

Angular unconformityyg

- Bivalve - Valanginian-Barremian-Inoceramus murgalensis

-Inoceramus peltiformis pochialaynen

B

- Ammonite - Cenomanian

- Dinoflagellate - Coniacian-early Campanian

MICROFOSSIL OCCURRENCES

- Bivalve - Turonian-InoceramusL

ate

KL

ate

J -

Ear

ly K

Lat

e Tr

- E

arly

J

D i s c o n f o r m i t yU-Pb AGE SAMPLE LOCATION

m s gf

Volcanic breccia

Chulitna terrane (Limestone, basalt; qtz.-rich redbeds;volcaniclastic redbeds; marine sed)

Argillite, chert, and tuff

Conglomerate

Lat

e K

Lat

e Tr

KJ

Condensed stratigraphic interval

Condensed stratigraphic interval

isD sconformityDisconformityDisconformityDD onformityonfcoisDD forrmityymityrmforonfscoisDD

Disconformity

?

R - Radiolarian - Callovian-Tithonian

?

??

?

DisconformityfornfcoiscD miityDisD scoonfforrmmityy?

?

?

?

?

? Condensed stratigraphic interval

?

? Condensed stratigraphic interval

R

FIGURE KEY

Figure 20. Stratigraphic regional correlation diagram of Upper Triassic–Cretaceous strata throughout south-central and southwestern Alaskaalong the inboard margin of the Wrangellia composite terrane. Composite stratigraphic sections were constructed from references cited infigure; thickness of stratigraphic units is not to scale. Note that most elements of the three-part stratigraphy documented in the northwest-ern Talkeetna Mountains are present in each of the composite sections. See text for more discussion. Upper Triassic–Jurassic Chulitna stratig-raphy summarized from Jones et al. (1982), Csejtey et al. (1992), and Clautice et al. (2001).

Pre-, syn-, and postcollisional stratigraphic framework and provenance of strata 433

very similar to volcanic, volcaniclastic, and chert-rich sandstoneand conglomerate described from the Early Jurassic condensedinterval of the Honolulu Pass formation in the northwestern Tal-keetna Mountains (Fig. 20). A disconformable contact existsbetween the Tatina River volcanic unit and the overlying LowerCretaceous Kahiltna assemblage. This contact may represent aperiod of little to no deposition during part of Early, Middle, andLate Jurassic time (Fig. 20; Kalbas et al., this volume).

The Kahiltna assemblage in this region consists of inter-bedded mudstone, siltstone, and sandstone with minor conglom-erate and is interpreted to have been deposited in submarine fanenvironments. These strata are Lower Cretaceous (Valanginian–Albian) age based on both the occurrence of the bivalve Inocera-mus (Fig. 20; Reed and Nelson 1980; Bundtzen et al., 1997) andmaximum depositional age from detrital zircons (Kalbas et al.,this volume). Sandstone compositional data from the Kahiltnaassemblage in this area indicate predominantly arc provenance;limited U-Pb detrital zircon data indicate a mixture of arc sourcesfrom the Wrangellia composite terrane and continental marginsources of the Yukon-Tanana terrane (Kalbas et al., this volume).Shallow marine and/or nonmarine strata overlying the submarinefan strata of the Kahiltna assemblage have not been reported fromthis area (Fig. 20).

Colorado Creek/Chulitna RegionTriassic stratigraphy in the south-central Alaska Range

(Figs. 19, 20) is well exposed in the Chulitna terrane of Jones et al.(1980) and consists of Paleozoic limestone and fine-grained tur-bidite strata, and Upper Triassic pillow lava, fossiliferous lime-stone, volcaniclastic strata, and a quartz-rich redbed unit (Jones etal., 1982; Csejtey et al., 1992; Clautice et al., 2001). A Norian agehas been assigned to Upper Triassic strata based on the occurrenceof the bivalve Monotis and hydrozoan Heterastridium (Blodgettand Clautice, 2000; Clautice et al., 2001). Upper Triassic strataare overlain by a succession of Lower Jurassic sandstone, lime-stone, and argillite that is thought to be as young as Early Juras-sic (Sinemurian) in age based on the occurrence of ammonites(Blodgett and Clautice, 2000; Clautice et al., 2001). These strataare partially age equivalent to the Early Jurassic condensed strati-graphic interval described from previously discussed locations inthe suture zone. Some workers have suggested that the Upper Tri-assic pillow lava and limestone stratigraphy of the Chulitna ter-rane is time-equivalent and bears some lithologic similarity to thatof the Honolulu Pass formation in the northwestern TalkeetnaMountains and to the Wrangellia stratigraphy exposed south of theTalkeetna fault (Fig. 1A; Jones et al., 1982; Csejtey et al., 1992;Clautice et al., 2001).

A poorly defined unit consisting primarily of argillite andcherty argillite with minor cherty tuff and basaltic tuff is thoughtto overlie Early Jurassic strata and may be as young as Middle–Late Jurassic (Callovian–Tithonian) age based on radiolarianbiostratigraphy (Blodgett and Clautice, 2000; Clautice et al., 2001).The relationship of this unit with the overlying Lower CretaceousKahiltna assemblage is unclear; however, if it represents the base

of the Kahiltna, a disconformity representing part of Early Juras-sic and Middle Jurassic time may exist between this unit and theunderlying Upper Triassic–Lower Jurassic (Norian–Sinemurian)strata (Fig. 20).

The Kahiltna assemblage in this area is Lower Cretaceous(Valanginian–Cenomanian) and consists of submarine fan depositsof interbedded sandstone, siltstone, and conglomerate with minorlimestone (Eastham, 2002; Eastham and Ridgway, 2002). The ageof the Kahiltna assemblage in this area is based on the Early Cre-taceous bivalves Inoceramus and Buchia and Late Cretaceousammonites (Jones et al., 1980; Jones et al., 1983). Sandstone com-positional data from the Kahiltna assemblage in the central AlaskaRange have a mixed arc and recycled orogen signature (Easthamet al., 2000). Conodonts from limestone clasts in conglomerate ofthe Kahiltna assemblage indicate derivation from Paleozoic con-tinental margin strata (Ridgway et al., 2002). U-Pb detrital zircondata from the Kahiltna assemblage in this area show a clear mix-ture of sediment derivation from the Wrangellia composite terraneand continental margin strata (Hampton et al., 2005). Overlyingthe Kahiltna assemblage with an angular unconformity is a thinsuccession (�20 m thick) of Upper Cretaceous (Coniacian–earlyCampanian) strata that contain marginal marine dinoflagellatetaxa (Trop et al., 2004). These strata consist of interbedded sand-stone and mudstone that disconformably underlie lower Oligocenenonmarine conglomeratic strata (Trop et al., 2004).

Correlation Summary

Regional stratigraphic correlation of Upper Triassic–Cretaceousstrata throughout the Alaska Range suture zone in southwesternand south-central Alaska demonstrates several common stratigraphicelements. The first common element is Upper Triassic–LowerJurassic volcanic and carbonate strata that appear to have compa-rable lithologies with similar fossil occurrences (Fig. 20). Withthe exception of the Upper Triassic siliciclastic redbed unit of theChulitna terrane, Upper Triassic–Lower Jurassic strata through-out the suture zone share closest commonalities with volcanic andsedimentary rocks described for the Wrangellia and Peninsularterranes. Within the context of the tectonic evolution of the suturezone, we tentatively interpret the Upper Triassic–Lower Jurassicstrata of the Lake Clark region and Tatina River area as represent-ing precollisional deposition on an arc margin and related carbon-ate platform/ramp as described for the Honolulu Pass formationof the northwestern Talkeetna Mountains. Much more work isneeded to determine the nature of the units that underlie the UpperTriassic strata throughout the suture zone in order to test our pre-liminary correlation.

A second common element documented in our regional cor-relation is a hiatus in the stratigraphic record that may represent acondensed stratigraphic interval and/or disconformity that occurredthroughout part of Early, Middle, and Late Jurassic time. This is bestdocumented by what we interpret as a condensed 75-m-thick strati-graphic interval representing up to � 25 m.y. of limited sedimen-tation during Early Jurassic time and a � 20 m.y. disconformity

434 Hampton et al.

that lasted throughout Middle and part of Late Jurassic time in thenorthwestern Talkeetna Mountains. The importance of this con-densed stratigraphy and overlying disconformity throughout thesuture zone is currently not well understood. Within the contextof the development of the suture zone this event may record ther-mal subsidence or drowning of an inactive volcanic arc/carbonateplatform. A second hypothesis is that this event is the product oftectonic subsidence due to loading of the arc margin by someunknown (at present) tectonic feature.

A third common element in our regional stratigraphic corre-lation is that the Late Jurassic (Kimmeridgian) marks the initiationof submarine fan deposition throughout the suture zone (Fig. 20).In the northwestern Talkeetna Mountains, this stratigraphic pack-age coarsens upward and deposition continued through at leastAptian time, but locally in other parts of the suture zone subma-rine fan deposition extended into Cenomanian time (Fig. 20). Inthe context of the tectonic evolution of the suture zone, we inter-pret the thick Upper Jurassic–Lower Cretaceous submarine fanstrata to represent the syncollisional sedimentary record associ-ated with collision of the Wrangellia composite terrane withinboard terranes of the continental margin. Our provenance datafrom the Kahiltna assemblage in the study area clearly show thatsediment deposited in the Kahiltna basin was being derived fromboth the Wrangellia composite terrane and the Mesozoic conti-nental margin.

A fourth common element in the regional stratigraphic corre-lation is the transition from Upper Jurassic–Lower Cretaceousdeep marine, submarine fan strata to overlying Lower Cretaceousand younger shallow marine to nonmarine strata (Fig. 20). Creta-ceous strata in the northwestern Talkeetna Mountains, the newlydiscovered Caribou Pass formation, are nonmarine and werederived from exhumed continental margin strata of the Yukon-Tanana terrane based on our compositional data. In the centralAlaska Range (Colorado Creek/Chulitna area), partially equiva-lent marginal marine strata overlie the submarine strata of theKahiltna assemblage with an angular unconformity. Potentially,coeval shallow marine and minor nonmarine strata of the Kuskok-wim Group depositionally overlie the Kahiltna assemblage in south-western Alaska (Fig. 20). Although additional study is needed todetermine the nature of the contact between Upper Cretaceous andunderlying strata throughout the suture zone, the presence of non-marine to shallow marine strata and isolated angular relationshipssuggests that Upper Cretaceous strata represent the transition froma pre- to postcollisional stage of suture zone development. Collec-tively, our correlation suggests a southwestward nonmarine tomarine transition in interpreted postcollisional strata.

In summary, the three-part stratigraphy established by ourstudy of Mesozoic strata in the northwestern Talkeetna Mountainsappears to be correlative to similar strata along strike in the suturezone. Regional correlation between four areas over a distance of�375 km in a structurally complex suture zone is overly optimisticat best, but we feel that the stratigraphic similarities exhibitedthroughout this region are strong enough to warrant additionaldetailed stratigraphic studies that have the potential to help delin-eate the tectonic development of southern Alaska and contribute

to a better understanding of the general stratigraphic record of col-lisional continental margins.

CONCLUSIONS

1. Newly defined stratigraphy for Mesozoic strata exposedin the northwestern Talkeetna Mountains consists of threeparts: the Upper Triassic–Lower Jurassic Honolulu Passformation, which represents volcanic arc/carbonate rampdepositional environments; the Upper Jurassic–LowerCretaceous Kahiltna assemblage, which represents sub-marine fan deposition in a marine basin; and the LowerCretaceous to younger Caribou Pass formation, whichrepresents nonmarine sedimentation. We interpret thisthree-part stratigraphy to represent the pre-, syn-, andpostcollisional stages between the Wrangellia compositeterrane and the Mesozoic continental margin based ongeologic mapping, measured stratigraphic sections, andprovenance data.

2. Detailed geologic mapping of the study area utilizing thethree-part stratigraphy shows that it consists of two north-west-verging thrust sheets. Our new structural interpreta-tion is that of more localized thrust-fault imbrication ofthe three-part stratigraphy in contrast to previous interpre-tations of nappe emplacement or terrane translation thatrequire large-scale displacements.

3. Peak U-Pb detrital zircon ages from the Kahiltna assem-blage show that deposition continued at least through EarlyCretaceous (Aptian) time. This new finding extends the agerange of the Kahiltna assemblage exposed in the north-western Talkeetna Mountains by a minimum of 10–15 m.y.

4. The provenance of the Kahiltna assemblage in the studyarea indicates that much of the detritus was derived fromactive and remnant arc rocks of the Wrangellia compositeterrane as well as from plutons located along the Meso-zoic continental margin of North America. Thus duringdeposition of the Kahiltna assemblage, the allochthonousisland arc terrane and the continental margin were in closeenough proximity for each to contribute sediment to theKahiltna basin. In addition, sandstone compositional dataindicate that there was a fundamental shift from arc-derivedsediment from sources located south of the study area tocontinental margin-derived sediment from sources locatednorth and east of the study area during Albian time.

5. The stratigraphy that we have defined for the northwesternTalkeetna Mountains appears to be correlative to stratigra-phy throughout the suture zone in parts of south-central and southwestern Alaska. Common stratigraphic elementsinclude: an Upper Triassic–Lower Jurassic volcanic base-ment that has some similarities to the Wrangellia and/orPeninsular terrane; a condensed stratigraphy and regionaldisconformity that may span up to ca. 45 m.y. during Early,Middle, and part of Late Jurassic time; Upper Jurassic–Lower Cretaceous deep-marine submarine fan strata thatwere mainly derived from oceanic source terranes; and

Pre-, syn-, and postcollisional stratigraphic framework and provenance of strata 435

Upper Cretaceous shallow marine and nonmarine stratathat were mainly derived from continental margin sources.

ACKNOWLEDGMENTS

This research was mainly supported by the U.S. GeologicalSurvey project Talkeetna Mountains Transect: Tectonics andMetallogenesis of South-Central Alaska, and partially supportedby the National Science Foundation and the USGS EDMAP Pro-gram. Peter Oswald and Eric Farmer contributed to field mappingand measuring of stratigraphic sections. Palynological analysis ofthe Caribou Pass formation was completed by Robert Ravn of AeonLLC, Anchorage, Alaska. Preliminary age determination of fossilleaves was aided by discussions with Scott Wing. Jonathan Glenand Skip Cunningham are thanked for discussion on findings dur-ing the transect project. Amy Draut, Sarah Roeske, and Jeff Tropprovided insightful reviews that helped improve the manuscript.We also have benefited from informal discussions with DwightBradley, Jeff Trop, Sarah Roeske, James Kalbas, Kevin Eastham,Michele Gutenkunst, Andy Cyr, George Plafker, Warren Nokle-berg, and many others. We especially thank Dwight, Lauren,Alice, and Dan Bradley for allowing us to use their home as a basecamp during the course of this project.

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